Thanks Peter,
well, I'm not in the position to make a test unfortunately.
Nevertheless your reasoning makes a lot of sense and actually matches exactly with what I had in mind.
I think I'll go for the fgz, although I haven't got a digital body, better be prepared for the future....

Many thanks to all of you.

First of all, many thanks for your contributions.
Well, this all started by reading the MZ-S manual, just to stay on the safe side. I came across this sentence:
"if you use different type combination of flashes, combine type C and type D or type E and type F" (page 100), being type A, the built in, type B = fgz, type c & d = ftz and type E & F = everything but ftz and fgz.
Apparently this is Pentax recommendation. Since the sentence was quite cryptic I chatted with Pentax USA support. Actually, after a long wait, they replied to my enquiry by stating that I was allowed to mix them only if I used them in manual, but no TTL.
This sounds quite awkward, I would have said no P-TTL but ok for all the rest.
Pwynnej, have you tried the combination?
Mannesty, I actually plan to use them with the dedicated F5P cords and Pentax hotshoe adapters, basically pure TTL, no wireless.
Hope this clarifies a bit.

Looking forward to your comments

Hello!
I'm new to this forum but a loyal Pentax user for ages.
Since I haven't been able to find consistant information on my problem on the web, I've thought that maybe some of you might have encontered the same problem.
I'm doing some portrait photography with two ftz500. After one died I wanted to replace it with a newer fgz540. Unfortunately it is said that it's better not to mix ftz and fgz, although my MZ-S is supposed to handle both series.
Can someone explain what is the problem and what would I lose by doing that?
Many thanks
Manlio