You shouldn't worry about the soundcard connected or whatever, it's fine.

And there never is 3.3v on an AT connector, only on ATX, so as long as the board supports your AT connector, it's fine.

In short terms, ATX to AT is fine

AT to ATX is not fine.

Thanks, I'm going to get this one so (even with the lack of molex, I'm just gonna get some adapters and gg).
I was afraid about the lack of -5v because I saw some testing vídeos and some isa sound cards are affected by not having it (so the fm sound sounds quite wrong).

Just be aware that FSP still uses crappy capacitors in most of their PSUs (especially lower end like this).
They will run fine for about 2-3 years and will not blow up afterwards, but you do need to check on them regularly.

Powerwise you're ok with the 90W this provides on the 5V & 3.3V rail. It would not be ok for a higher power Socket A system.

HELL
Well let me update the stuff:
-First, the motherboard is 20 pin and the psu 24 know only, so I cut some of the wires and voala it fits...
-And the worst part, the system doesn't even boot, the cpu cooler is a bit weak and it takes 10 seconds to turn it off.
-from many of my tries, only two resulte in system beeps (infinite beep) and in other case it showed the voodoo bios and nothing more...

Hell, this psu wasnt cheap 🙁
Now do you want to hear the bad joke? I just found a really cheap psu around here (it looks like a $5 psu) and hell, it worked...

Strange. It probably expects a higher load on the 12V rail.
Power supplies that bypass this issue for sure are the ones with DC-DC converters like Seasonic's G series.
20+4pins is also rarer, pick up an adapter to avoid cutting into the PSUs leads.