In terms of the inch per gallon rule, Yes you are overstocked, but that doesn't mean that you can't add anymore, but you have to be careful what fish you put in it and you have to make sure you have adequate filtration and maintenance.....
You bet I do. I have an emporer 400 on a 20 long. As far as maintance the only thing I use to put off was algae growth, but I bought an magnetic srcaper so its more easier now. Any suggestions?
Post your nitrAte level just before a water change. If it's below 40ppm, you can add more fish as your filters should be aerating the water enough as far as oxygen goes. However, do still keep the potential sizes of each fish in mind - your nitrAtes may be fine whilst the fish are small but they may seriously spike in future and you'll end up having to do twice-weekly water changes - or maybe even more...