^^^ Yes. This exactly.

I have never had my imagination be a crutch. You just have to be clever enough to work out the major details.

There is a reason for ANYTHING/EVERYTHING.

Though to restate my main point, cause I was kinda half asleep when I typed the above, is that having a reason, and having a good reason aren't always the same thing.

It comes down to the GM/player taking the time and effort to think a few steps ahead and invest the creative energy into their game/PC.

As long as that is willing to be done, it shouldn't be a problem.

I was gonna mention the RT Tau character guide but Crystal geyser beat me to it.

So why wouldn't a Tau fire warrior join a RT crew? We know of humans who have gone over to the tau side. Maybe this is a Tau who secretly disliced the caste systhem/ ethereal overlordship and joined the humans.

We are playing the introductory adventure that was released side by side with the Tau PC guide and it introduces it nicely - basically an inquisitor working with some Tau (and in our case because a PC wanted to play a Kroot, some Kroot as well) gets into some trouble in space and the RT and his crew have to come intervene.

Things happen. Then for whatever reason (we have not got there yet) my Tau PC and the Kroot PC end up joining the RT's retinue.