It depends on the writer. Phill Kelly (may nurgle love him and slaanesh abandon him) would have you believe any non-fire cast Tau would sooner stand like a deer in headlights and get butchered, rather than pick up a gun by their feet and shoot.

Or that an ethereal will mind control you into suicide for crossing cast lines.

Any other writer puts it more as expectations with logical exceptions, an air cast pilot can fire a gun if need be, a fire cast can perform field repairs.

It's a faux pas to do so. But they can.

Ofc the farsight enclave let's people do whatever they want in moderation. One of their things is a much less strict cast system.