how much work would it be to scale the armor pieces so the Zealot may get some new dress?
Making the zealot model wardrobe-able is a LOT of work I'm afraid. There are just so many armor meshes to refit to her frame.
Maybe one option to consider is using
@Kva3imoda 's "suits" method to make just a few armor suits.
Are there other models/monsters playable? I'd love to use some in my Destiny mod, maybe your Zealot too?
No question, you are completely free to use the files I put together for the playable Zealot. Here is the link to the unpacked MEDIA folder...
https://drive.google.com/file/d/0B2aVtkeuHrtcanR3Nl96bGZucTg/view?usp=sharingAs for
other playable monster models...
As I make any refinements I will make sure to give you updated links as well

1. Difficulty of
setting up the model files depends on how "polished" you want the animations to be - the simplest way to make animations for every move the model needs to make is to select a suitable existing .SKELETON file for each animation and then rename them internally so the game can "call" them. However there will be some animations which "look funny" and in some cases no animation at all (like the cannon animations problem for the Zealot).
There are some tools which were made to automate the re-writing of .SKELETON files, so that you can copy any animation to any model .SKELETON. These should be able to produce very polished animations, but I have not really worked with those yet to know.
And of course the ultimate polish (and difficulty) is to make new animations from scratch in a 3D modeling program and export those for use in the game.
2. Difficulty of
setting up the skills they will use depends on how unique you want them to be. I just made the Zealot use the Embermage skills so I wouldn't have to create new skills!

(the easiest option). The highest difficulty of course is making a complete set of unique skills for the monster class (the TL1CP mod took 2 years to complete!).
BUT if we have the "classless" concept working already, then maybe the class models can be "blank" in terms of skills - they will just "choose/learn" the skills as they progress through the game

3. And as mentioned above,
wardrobing is always high dificulty