I already had tau that my relative gave to me and I wanted to know if I could employ them as allies to the custom space marine chapter I'm making.
If you mean can you use them in the same army, no, you can't.
Your army needs to share a keyword, your space marine army's keywords would be the name of the chapter, "Adeptus Astartes" and "Imperial". None of these keywords are on any tau units, so they cannot be included in the same army.
8th edition did away with the allies matrix, keywords are all that matter now, and they do not share any keywords.