I can't see putting in an undergravel filter without removing all the gravel from the tank. You take the water level down 6", put a dish drainer or similar strainer over the tank, lift the gravel into the strainer by the cupful and dump the strainer into buckets.
Yes, perhaps even more so. UGs do a good job of biological filtration-they turn waste into nitrate. You still need to change water to get out the nitrate. If you already have a canister filter, such as a fluval, you can divert either the input or the output through the UG, but you have have trouble plumbing it, esp. as a 55 will need two separate filter plates, and thus two upright tubes. Running a filter output through a UG is called reverse UG filtration and will tend to push the waste out of the gravel. It might make for an uglier tank, but less gravel washing (you clean the waste from the canister filter instead). In a normal UG filter (air, powerhead or filter input) waste collects in the gravel and under the plate. Periodically, you need to repeat the remove the whole gravel removal process and lift the plates to clean under them. Oddly enough, most of my fish seem to tolerate this total destruction well and I haven't lost any fish from major tank cleanings, but cichlids are tough. Powerheads for UG filters should be fairly weak, after all they are replacing air-stones. If it was your only filter you would want a high flow rating , but the filter makes a worse mess if it is strong enough to suck food down into the gravel before fish eat it. A lot of people don't like UG filters anymore even through they are tried and true. Giant sponges with power heads can give you equivalent biological filtration with less work. If I had lots of money I'd replace all my UG filters with external sumps that I could clean by back flushing into a sink instead of breaking down the whole tank.