If a set of cranks need a 104mm bcd will any 104mm bcd rings fit? So FSA fit on Shimano cranks and vice versa?

Most will fit, however with certain brands the inside tabs (Bit below the bolt hole) can foul the tabs on the cranks. So either check with who you are buying the parts off or buy a dremel multi.

IIRC you should be fine with most chainrings on shimano. Raceface on the other hand can be a little awkward to fit on.

Some of the more recent shimano carbon style rings (SLX, XT etc) have a thicker, more rounded tab – so wont fit other chainsets.

I’d be fitting a Shimano middle ring to an FSA chainset (Shimano is cheaper) they’re both 104mm so I’ll risk it for a biscuit.