First off, sorry if this will sound like a complain, belive me it's not and I know programing can be hard :(
I really liked the mod's idea tho, it's a cute little fellow that clar some of the screen polution and make characters like Lilith OP. But given to some comments and for my own testings, I have few ideas.

1 - Makes it a chance to spawn (not guaranteed when you have 2 or more familiars), maybe give a 25% chance to spawn when you have 2 familiars and increasing (about... 5%?) per another one you have above 2.
2 - Would be nice if it wasn't rerollable, if you use a D4/D100/dice room it'll be rerolled like a single item.
3 - There's a bug when you reroll the plush, since it's "gone" from your iventory, the sprite basically slide around slowly till it leaves the screen and resets in a new room, only being fixed in the next floor when you find another plush (if you have 2 or more familiars after the reroll), making it have a "follow" command even without familiars would maybe fix that, and like I said on item 2, making it unrerollable could fix any other possible issue
4 - there's still a small bug with the charge bars (even the vanilla ones added in game after those expansion packs) where if you have multiple charging familiars theyr bars will apear mid-screen and will not move at all, maybe making them follow behing the plush (like if you still had a "chain" of familiars).

anf finally, sorry if I was "annoying" in here. Wish I could do more and help fix the issues, but Im no programmer... But I really enjoyed the idea of the mod and hope to see more updates to it, new animations to the plushies, new plushies, idk, I just hope to see more of this mod, keep the nice work man :D