Allowed Custom Player Models

[Unh3lpful]

What models are and aren't allowed as player models? I've been trying a few and it's been hit and miss. The robot models work, as does some strange pyrovision model called 'kitteh' but others don't such as the barrel and a few cosmetics I've checked.
Is there some tag or item I need in order for it to work and if so is this something I can alter?

Thanks in advance,

Unh3lpful
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EDIT:
Also, if anyone knows how to make player cosmetics invisible that would also be appreciated. I've tried making the player invisible and the model did become invisible but cosmetics remained opaque. I also tried using a tf_condition thing and making the player cloaked but that didn't work either.