Is the Farsight Enclave purely tau or are other races such as kroot and vespids allowed to join the Farsight enclave?
According to the WarHammer 40k wikia, there are... humans in the Farsight Enclave on the world of Gue'vesa'rio:
Gue'vesa'rio - Gue'vesa'rio is a world in the same star system as Salash'hei. As its name implies, it is home to the Gue'vesa population of the Enclaves, those humans who for various reasons have turned [their] back upon the Imperium of Man and pledged themselves to the service of the Greater Good.
There is no source for this precise statement, but the article uses the Tau Codices for the 3rd, 4th and 6th editions as its sources, as well as the Farsight Enclaves supplement. It seems that the source is the latter, if I understand the German version of Lexicanum well.