"I have followed the myriad potential futures of the Tau with great interest. Though barely even striplings compared to us, I feel a strange protectiveness towards them. In time I believe they will exceed even our greatest feats and master the darkness within their souls."
Although it still doesn't change the matter. They can get along and live close together just fine, but as others have said, they would only ally with them if it was a benefit to themselves. Example, using the Tau to slow the approach of a Splinter Fleet.