What did the Tlaxcalan's hope to gain from the alliance and how did the Tlaxcala see themselves in the post Aztec order?
You may be interested in my earlier answer on a similar topic:
In the second part I look at Tlaxcala's "special status", so you can also go straight to this - this focuses more on colonial times and Tlaxcala's rewards for siding with the Spanish
This answer to a follow up also directly deals with Tlaxcala in more detail (esp. on the 2nd question) - here I look more at Tlaxcala's reasons for forming the alliance in the first place
Hope that's helpful! In case of questions let me know.