There are a few issues at hand here.
1) Gaining. Set the crossfader to center or better yet switch it off (if you are not scratching you really don’t need it LOL). Proper gaining of your system is important. Load a track in a deck and play a louder part of it. Set the channel gain/trim so the loudest part has the 0dB light burning (near)constantly and the next light flashing on the peaks. Now open up the channel fader to full. You are now putting out a constant 0dB signal to the main bus. Turn up the master volume knob until the master signal meter reads a constant 0dB with one light more flashing on peaks. Take a piece of white gaffa tape, stick it next to your master volume knob and mark the setting.
Next set your speakers to their respective 0dB settings (usually marked), if not, just set them to full. As you can never send more than a 0dB signal to the speakers, they should not go into limiting. If they do, just run them up til it goes into limiting and then set them back a bit (about an 8th of a turn or so). If you keep an eye on your channel trims with every track (best practice anyway), you will never clip or oversteer anything.
2) Speaker setup. Running your speakers (too) hard because you have more people than the system is made for is not good practice. It will make things sound crap and worst case kill your speakers/amps.
Proper way to set up a single sub/two top configuration is (generally, assuming the sub has both input and high pass top outputs) to run dual XLR cables from L+R output on controller to sub. Then run a single XLR from the high pass outputs to the individual tops.
If you add a second sub (assuming same brand/type), you just unplug two XLR cables(input sub and high pass output) and run the input into your second sub and the high pass output from that sub to the other top. Making for two identical stacks on either side.
Do not mix and match if you can help it. If you want to add a complete system (sub with 1 or two tops) from another brand (a matching set, so not a WD sub with a Mackie top), just use the full signal through outputs on the sub and run it to the seconds system.
If you want to use various brands of subs and tops together, daisy-chain the subs from your main sub (no stereo image there, so just go from one to the next) and the L and R tops from the through output of your main system tops.
Clearly you want to be using proper XLR cabling for all of it.
3) Indoor we calculate an average of 5Watt RMS per person, outdoor about 10W, all depending on musical styles too. If you play much bass-heavy stuff and you want it to be felt, you can go to 10W/person indoor. If you have 700 people at a gig, you’d need at least 3.5KW and up to 7KW (although that’s a bit overkill imho, even for 700 people). I can safely assume that your Alto setup won’t make that (or anywhere near). I have a Mackie HD-series setup with 2x 12″ tops and 1x 18″ sub and it puts out true 2kW RMS. So I stay under 350 people to be on the safe side. Any more than that I either rent extra sub/tops from Mackie HD-series or I just get a complete rental PA for the occassion. It beats having several brands and types and all.
Also for a bigger venue, I would always choose to have multiple tops spread through the room (even considering going mono if my setup allows it) to have a more even sound level dispersion and to prevent things from being way (too) loud at the front and not loud enough halfway and further down the room. I usually go 4 inward firing speakers on the corners and even a set of side fills halfway down the side (also firing into the dancers) if the room is relatively long. This way you can run your speakers a lot less hot and still have a nice even sound all over the dance area.
Hope that helps some.
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