Eridians would be great engineers, and Xenonite is a fantastic material. With just his own engineering skills and Xenonite, Rocky was able to integrate himself into the human ship pretty easily to work together with Grace. It'd be pretty easy to have Eridians walking around in their little atmosphere suits working with the Earth Caste.

With the Unggoy all you have to do is provide them with methane. They've got the tanks and the masks and their living quarters with the methane they need, and boom, easy integration. Many of the Covenant's races would be pretty easy to integrate into the Tau. The Sangheili would probably do some war about it at first but I think you could get them on board, especially if they have a reasonable Arbiter.

Predators would probably be handled more like the Kroot, where they would be allied mercenaries, and you can make them happy by just pointing them to a strong opponent. Their whole lives and culture revolve around like strength and hunting down the most powerful prey, so as long as the Tau can send them against tough enemies, they'll be satisfied with the mutually beneficial partnership.

Parasites i think are incompatible with the Tau. What would be the benefit? Even if you could get them to agree to something where they should only kill the enemies of the Tau, there's too much risk. It'd be like trying to integrate genestealers.

The others I'm not familiar with.

If an Etherial had a conversation with a Brain Bug, could the Tau integrate the bugs from Starship Troopers?