You can hook any speaker up to a power amp, as long as you match impedances and don't turn up the power amp so much that you blow the speakers.
Okay, so if I was going to hook up some cabs to a power amp, I would first look at the impedances involved. Let's take the link below as the power amp in question, and the second link as the cabs in question. I think this cab kinda sucks, but it is a stereo cab, and it will help give examples.
So the power amp -
"...pumping out 150W RMS per channel at 4 ohms, 100W RMS per channel at 8 ohms, and 300W RMS mono at 8 ohms..."
And the cab - " ...Power rating: 400 Watts into 16 Ohms mono, 2 x 200 Watts into 8 Ohms stereo.... "
Okay, so power amps can be operated one of two ways - stereo or mono. Stereo means L and R in, L and R out. Mono means one in, one out.
Speaker cabs can be either in only or can be parallel - not many guitar cabs have the parallel option, it's something more common in bass cabs and PA speakers. This would mean you could hook many of them in a row... but we'll skip that, that's another question.
So let's explore our options. The cab can be hooked up in mono at 16 ohms/400 watts, or stereo at 8 ohms/200 watts each channel. The power amp gives 100 watts at 8 ohms per channel... so if we had two inputs we could take 1 power amp and hook up one cab in stereo - 100 watts per side. Nice and loud. Here's the thing, though - we can't hook the cab up in mono, because its not rated for 16 ohms. I mean, we could, but we would get less than 100 w of power. So we would pretty much be limited to 1 power amp -> 1 cab, since we'd have to use 1 power amp to power each side of the cab.
As ohms rise, the amount of watts decreases. So an amp could be rated for 1000 w at 2 ohms, but by the time you get to 16 ohms you could have as little as 200-300 watts of power available!
Umm, maybe that wasn't as helpful as I thought it could be. Well, when you bridge a power amp you take two channels and turn it into one (ie bridging, there's usually a switch on the power amp) ... so one channel comes in, one channel leaves. Doing this increases the available power... so we might be able to bridge this amp and run a 16 ohm cab, although keeping in mind that we'll probably be getting less than 200 watts of power by doing this. It is a valid way of doing it, though, and would make it easier so we wouldn't have to screw with splitting a mono guitar signal into two stereo signals.
Does this all make sense? Hope it does.
It is perfectly okay to play a guitar through a PA system, although you want to keep in mind that if you aren't using guitar speakers you may need to have some form of speaker emulation - guitar speakers roll off a lot of the noise that amps make, so they function as natural filters.
If you're using a multifx pedal or amp and going straight into a PA, then you probably don't need a speaker emulator - multfx and modelling pedals/effects/preamps whatever tend to do better with a wide frequency response, ie, not necessarily using guitar speakers.
Good luck!
Saul