Reused Boss models/skeletons you've noticed.

[MariaArvana]

It may just be me, but I think Eden Ifrit is based on Diabolos with the wings removed, and Eden Garuda reminds me of Scathach. Which, I suppose, foreshadows their fusion (Raktapaksa/Diabolos Hollow).

nah, Eden Garuda's & Scathach's animation skeletons aren't even remotely close. Scathach's is used for pretty much any slim enemy model that holds some sort of spear and floats (Like Penithsilea in Eureka Pyros, and whatever the heck the add in A11 is called). If anything, ifrit is closer to Scathach, but he's also not using her skeleton. (it is similar to a wingless Diabolos though.)
Eden Garuda's skeleton is actually much closer or the same as Diabolos Hollow, as they have almost identical auto attack animations and extremely similar movement and idle animation.

Raktapaska literally uses Eden Ifrit's model & skeleton, only adding wings. (Watch his idle & auto attack animations.)

Pazuzu also uses the Diabolos Hollow skeleton
Omega is a re-skinned giant toad (watch the head movement)
Avatar from T8 is a reskinned Treant
Louhi is a reskinned Cruise Chaser
Demon Chardanook is a reskin of a wraith/phantom model
Chaos uses Sephirot's 1st phase skeleton
1st boss of Delubrum is a reskinned Ravana spliced with some Gilgamesh animations

E10 boss (on all fours) uses the same general 'behemoth' skeleton that's shared by tons mobs in the game.
While not an actual model swap, the E11 boss uses a huge amount of skill animations from GNB for its attacks (for obvious reasons)
All the SHB S ranks are re-skins of various bosses (Possibly the only exception being the Rak-tika one)

[Iscah]

All the SHB S ranks are re-skins of various bosses (Possibly the only exception being the Rak-tika one)

Rak'tika is the almost-a-boss "Ronkan Dreamer" from the first section of the Qitana Ravel.