Xweetoks can wear any item that is not specifically for a different species.

Here are the database items which are in the "custom clothes" special category. As long as it doesn't say a different pet (e.g. Bionic Cybunny Cranium Cover) a Xweetok can wear it.

Here are the 8 items which can be worn only by a Xweetok.

And then there are the paintbrush clothing items. They are listed as "unobtainable" in the database as you cannot sell or trade them.