Yeah, both of those monster types are incredibly hated, but...

The reasoning for new ones is because there are some "gaps" that need filling...

See, Gremlins/Contructs get the Mender... He has an annoying habit of jumping out of the way very fast
and Fiends/Undead get the Silkwing... He has an annoying habit of being really tiny and doesn't stop healing things.
Beasts/Slimes get nothing. meaning that instead their healing is covered by one of the above two if it happens to spawn near them.

The last "pairing" should get a healing critter too. I can't think of anything that's really fitting, but I keep imagining a Chromalisk that heals/buffs enemies by spitting goo at them. Plus it can go with the Chroma line's Invisibility as it's "annoying" tactic.

Or maybe OOO can revamp the Soul Jellies as Healers. I mean, the Slime family already has three or four rare/"special" monsters (The Imposto, the Sloom, and the Rock Jelly), they can afford to lose one to make it a healer. The only problem is that then you get 2/3 Healers who fall over when hit with Shadow damage.

As for the "turret"... Pirhana plants, clearly. The Polyp's weak to Shadow and the Howlitzer and Gun Puppy are BOTH weak to Elemental (which makes the Howlitzer a tad redundant even if it looks cool and fits the theme of the places it spawns in better than a Gun Puppy... then there's it's "last ditch skull missile" tactic which turns it into a slightly less annoying Rocket Puppy). A Pirhana plant (Beast type, clearly) would probably make up for those.