i think that the last officially supposed OS for FCP7 was 10.6.8
but who'd be using that still ?!
i've found 10.8. to be perfectly workable, as have you, it seems.
and I've got it running on a few machines that are Yosemite, 10.10.5
it's stable there, but there are a few small things that don't work.
Colour Picker is one, but if you don't do heaps of colour work it's not a big problem.
the AVCHD files have to be prepared for FCP, i think.
SONY have software that can do that.
sorry, I'm not very good with this side of things.
nick