Heya. Very vivid description, I like how you gave it a kind of cute appearance with predatory undertones. I do have a few suggestions as to how you might better present them, and ideas that mgiht help you flesh them out.
I also recommend you work from the inside out. Start with a specific colony, see how you end up writing them, and then build outwards.
--WRITTEN DESCRIPTION--
The way you describe your Pluizig is kind of like a chimera. You focus a lot on the different animal extremities. As a reader its difficult to put these parts together in my head as I'm focused more on the parts than the whole. I recommend trying to generate a short 3-4 sentence long description that makes it easy to visualize without too much reading. It sounds like a fun creature but you don't want to make your readers work to figure out your vision. Readers are lazy.
1) Start with the body-type. Give us one familiar animal to start with. Its a "fluff-ball" so is it bulky and bottom-heavy like a bear? Though you do mention cat-like forelimbs and dragon-like hind-limbs, so it might also have a sleeker feline body underneath all that fur. Start there.
2) Now mention the feathers, talon-like feet, and avian/theropod head. So now you can easily describe it as a "cat/bear-like creature, the size of an elephant, with shaggy fur and hawkish features" or some-such.
3) Add one to two more unique details. Maybe the horns and feather patterns? Make it simple. You don't want to give it "dragon horns" or "kirin horns" and have people needing to use google to figure out which interpretation of those magical creatures to imagine.
4) If you want to change the bird-head out for a raptor's... keep in mind when people hear raptor they think Jurassic park. If you want to give it more of a theropod-look you may want to re-frame it as a kind of fuzzy dragon.
5) I notice you mention a lot of different variations. Some have three tails, some have wings, and their plumage varies greatly by climate. I understand your reasoning in saying "anything goes" - it makes your species more open to use by others, and leaves room for growth in the future. But I suggest you be decisive and assign features which might be adaptive and fit their way of life. You'd expect to see some kind of divergent evolution or sexual selection with respect to wings, tails, and plumage. Do girls like big horns? Do the males want winged sons that would be great hunters and spurn their wingless offspring?
---HISTORY AND CULTURE---
So the rider selection idea seems neat and I definitely want to know more about your pact. These are a couple of questions I have which might help you flesh their society out further.
Why was this pact formed? Is this between a specific colony of Pluizig and a specific city? Do some resent having their children taken away to be reared as beasts of burden? And what sort of magic do they have? Why ride one in world's filled with dagons and wizards?
I hope this helps! I look forward to your final Result!