I have read over your post, and I think you are getting the 20 gal. from your friend.
If this is true, I would recomend Mbunas. They are good hardy fish. They are agressive, and actually do well if over-stocked. I would recommend about 5 or 6 of them in your tank. Provide plenty of hiding spots. I use two small pieces of slate to form a teepee or A-frame, along with various rocks and slates to form caves along the back. this leaves plenty of swimming room across the front. Mbunas are available in many MANY colors and patterns. You can have a very cool setup with these. They eat anything about the size of guppies, but not too often as they can't have too much protien/fat. Also, they are very easy to feed with a stable of cichlid pellets, and any number of frozen/freeze-dried foods, and blanched veggies. Remember a varied diet is the best for all fish. The fact that you want agressives, means that you WILL have to do water changes regularly. Mbunas give you the variety of having a larger # of fish in the same body of water. The only drawback, if any, is this does mean you will have more waste. But this is greatly surpassed with the enjoyment of these cool fish. I have 7 in a 29 gal. and will probably be adding one more.
I have a Penguin 330 on it. This cycles the water 11+ times an hour and has two biowheels. This benefits the overstocking, but I still do water changes about once every 2 weeks, but monitor the water closly. And it cost me less than $40. I would recomend any filter with some type of Bio-filtration. I have heard that Whisper has a filter with a flow gate giving a sponge access to oxygen and works about the same as bio-wheels. I haven't seen these in action and haven't had any experience with them. Beyond that, you just need a heater, gravel cleaner/syphon tube, themometer (don't trust heater settings), and water.
I love cichlids and really love my Mbunas. They aren't expensive (most aren't) and they're easy to start off with for agressives.
Oh I almost forgot, these are mostly top/mid water fish. This allows you to add a cool cat or other bottom-dwellers. I would recomend more than 2 or so. Also don't worry too much about the setup. These fish will pretty much like anything other than plants. They dig a lot, and I use filter gravel. It gives a nice natural look, and doesn't look as bad when they dig.
Good luck!!!
