alright, been doing some testing and research on the summon skills as best i can since theres no data on them ingame. im also doing live testing as i play,

generally speaking, summons do scale off /something/. i dont know what all it scales off of, but the stated health on my summons has gone up as i've leveled, presumably to stay more or less stable in strength compared to whatever it is you 'should' be fighting at your level. also, i noticed something odd in the various summon skills, namely a spell attribute stating apparently that having summons increases your willpower generation rate? havent tested it, and it doesnt show up under my personal stats so i cannot confirm if thats what it does, or if it has any real effect. be interesting if it does however.
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anyway, i'll essentially go skill by skill with tips. your builds are your own and i dont have any data on the passive nodes benefiting the summons.

Feeding Swarm
summon individual zombies up to cap, melee, reasonably fast but squishy beyond all scope and reason. you are going to have to semi-constantly replace these, personally speaking that stopped being as much of a problem when i got the health boosters for them, though its the difference of only an additional hit or two. currently (level 28) mine say they have 864 hp apiece. this is roughly half my very squishy casters HP, and i die /fast/ any time something actually gets a swing or two in against me. dont expect them to save your life or add much in the way of damage unless you have something else to distract the enemies away from these things.

Hunting Swarm
this one is not listed on the wiki, i only found out about it when i got it. havent yet used it though im about to start doing so for testing purposes. theoretically speaking, them being ranged means they should stay up easier and deal more consistent damage again provided you have something else to tank for them (either another summons or yourself). i cant give much more information on it at this point, i'll likely update this after i do some testing.

(update) exactly as i assumed. they are easier to keep alive and a good (if minor) addition to damage output. with the proper skill modifiers they should be a reasonable way to add damage. better at it than the feeding swarm anyway
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Livor Mortis
nominally your tank. in my experience this thing is rather sturdy, especially with the health nodes. mine is currently sitting at 30k hp (level 28 skill). its a nice solid single unit summons, and you can even swap what role you want out of it by using ravenous reach to turn it into a mobile fighter (at half health), or a support mage at a third health. if you only use a single summons, my experience strongly suggests that its this one. the aggro draw however is a bit spotty, but can be counteracted to some degree by liberal use of its taunt. just be very aware that the taunt range kind of sucks as far as i can tell, so the monster in question has to almost be on top of the thing or it wont work.
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Parasite
hoo boy... this spell. ok, this spell doesnt quite know what it wants to be. i will first off state i apparently kind of broke the spell by dumping for an insane amount of wisdom ASAP. im currently sitting on roughly 50% on all status ailment chances, so parasite builds status stacks FAST. the base damage on this skill far as im aware sucks. most of the damage i get off it is just sheer stack abuse, but allows me to drop bosses within seconds of channeling... and after building up those stacks i can GTFO instead of continuing to channel. the lifesteal aspect of the spell is terribly useful and is a good offset to the fact you have to stand absolutely still while using it. one big drawback to the spell is the overall cast time coupled with having to select a 'valid target' when you begin your cast. once you start though its locked on and will just stay on the target basically no matter what. so its an okay spell overall and insanely good if your building to abuse status ailments. now with that out of the way, on to the relevant bit; summoning.

as a summoning spell, parasite is confusing because it in no way indicates what it does or how it works. the only statement is if the laser beam of doom kills something, it will convert that unit into an allied unit. this is however a somewhat complicated mechanic, and a mildly buggy one in its AI. alright. first thing i will state is that most of this is conjecture. it holds up to my testing and experience, but it is a series of assumptions nontheless.

Parasite works off a point system of some kind. best estimate i have for the point total is 4. the source of that is if you grab the smallest crap mobs, you can total get 4 of them before they start dying off when you claim another one. somewhere along the line however, the point value of mobs increases. i use size as a scale, though theres enough variance to that to make it highly uncertain. basically if you get a mid-sized unit that doesnt have much in the way of special abilities, you can generally get two of them. or one of them and two of the dinky units. the units that i would highly suggest getting however are the big special units, which as far as i can tell always take all four slots. things like the Tormenters, Specters, and the big soul beasts that teleport.

beyond just grabbing big units, theres a few that deserve a special mention and lots of focus and love any time you can get them. Necromancers and skriir mothers i think they are called. basically, if you parasite a unit that can spawn units, you get them AND their spawn. with a necromancer, the spawn is unbelievably slow, but you can get functionally an infinite amount. with the mothers though, not only do they fire the spawn at enemies directly, but the spawn themselves are relatively quick. catch on them is that they can only have 6 at a time no matter what while parasited. there is however a problem with spawning units, and that is that the AI seems to break down over time and they just stop spawning creatures. so you will have to replace them even if they survive for an extended period, but meanwhile you have a disposable army available. which i assume is why you wanted to play summoner in the first place, so cheers lol

other than the spawning units, the others that i mentioned above are my go to whenever i get a choice. generally i will prioritize spawners over those, but anything large with ranged and AoE is useful, especially tormenters as they can heal themselves with that AoE. and final priority thing is i /think/ if you consume a special unit, it keeps the buffs it had before. theres no way to check, but it at least felt like Tough units i infected survived longer even if they were melee.
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anyway, thats all for parasite, and i dont have any other summoning spells (leveling plague burst), so i will probably update this when i have something worth updating it with. have fun with your summoning builds, and hope this is helpful to you