Yea, that air should work fine. When you get a skimmer you can probably get rid of the air if you want to and It'll clear up your water from all the bubbles.
The number of fish you have really depends on how good your filter is. If you have the basic filter set-up, the general rule is one inch of fish per five gallons of water. So for your 55 gal tank, all your fish together should measure about 11 inches. So if you buy an 11 inch fish than that's all you should put in your tank. Or you could buy 11 one inch damsels.
If you add a refugium, heavy skimming, ozone, etc. then you can push this up a bit to accomodate more fish.
:happyfish