As far as maintenance goes, I try to do 50-70 % weekly water changes on all of my tanks. The goal is to reduce nitrates during these water changes and instead of testing water on all my tanks, I just do the changes. The fish are happy and growing, so assumption is that water quality is good. I also tend to overfilter, with air driven filters (sponges and boxes) on tanks 20 gallons and under, with HOB and canisters on bigger tanks. I use exclusively Aquaclear HOB's, as they do not require replacement cartridges that are just a reason for the manufacturers to keep selling you stuff. AC's have media that is easily rinsed with your water changes. For big tanks, I prefer FX5's and Eheim canisters, and try to clean them on a quarterly basis.
Depending on your substrate, a regular vacuuming of the mulm off the bottom is a good idea. I use sand in most of my tanks so that the stuff sits on top. With a gravel substrate, vacuuming is more important as the gunk will settle down in the gravel and can be a nitrate source if not cleaned regularly.
Not sure about your question on water level. I fill my tanks to the top. If you are running air into the tanks, that will provide plenty of agitation for gas exchange. I don't see a good reason why you would want to keep your water levels down unless you don't have a top on the tank and are worried about jumpers.
As for your stocking, I would do some research to see if you want to keep New World (which the parrots are hybrids of) or Old World (Africans). Pick one or the other for this tank, as they have different water chemistry requirements and different social structures. This isn't to say that you can't keep the stock the way you have it, and many have mixed similar fish in the past, but I think the fish thrive when they are separated. It will give you a reason to set up another tank anyhow. Once you've made your decision, research the fish you have (or want) so see how they are best kept. Most African mbuna are best kept in groups with 1 or 2 males and 4-6 females. NW are typically spawn in pairs.
Hope this helps and ask more if it doesn't.