Any advice/cautions on intermixing Z7U footage with the new FX1000?

It looks to me like I should be able to do multicamera editing in FCP with the unmodified HDV footage from both these cameras BUT I'd hate to spend $3k and find out that I'm wrong.

Just good old HDV capture as far as I'm aware. Shouldn't be a problem at all - I plan to buy the Z5 and mix with FX7 and Z7 footage. Easy peezy. Latest I've read is a November release date.

So HDV is HDV? No issues at all? No concerns about frame size matching for multicamera editing?

Correct. I've done shoots mixing Canon A1, Sony Z1, Z7 and FX7 all on same timeline... it's all 1440x1080. Not a problem.

So HDV is HDV? No issues at all? No concerns about frame size matching for multicamera editing?

Well just off hand... there is 720p30 HDV; 1080i60 HDV; 1080p30 HDV; 1080i50 HDV; 1080p25 HDV; 1080p24 HDV; 1080f30 HDV; 1080f24 HDV. Which ones am I missing?

But to get to your question, if you are using a few cameras that are all "XYZ" (X=480/720/1080, Y=i/p, Z=24,25,30,50,60) you shouldn't have any trouble editing them all together.

For multicam editing, you need to be more concerned with colors & time sync. Canon seems to be leading w/ the HDV multicamera sync work flow, but I think just about all Pro-line Sony cameras can sync timecode, & then half the battle is already won.