A larger ugf like a 55 works best with a power head on both sides. I use both powerfilters and ugf in the same tank and run them from different electrical circuits. If you do that then don't worry about the little bit of gravel that is not over the filter plate, I seldom use ugf under much more than 1/2 a tank anyway so I can do planting. A ugf is basically a big biofilter that uses your gravel as the media to support/shelter bacteria. It takes a little longer to get fully established than power filters, A 20g will work fine with just airlifts but larger plates need powerheads to get enough flow through the gravel though I did use several small seperate filterplates and airlift tubes in a 90g once, I just disguised the scattered uplift tubes with Java moss, looked pretty cool and worked fine. I use around 3" of fine-medium gravel and do a siphoning of surface detritus as needed, once a moth or so I stir up a small section of the gravel to make sure that it doesn't get compacted, if you have a community tank and feed tubifex worms fish like corys and clown loaches will do a lot of that for you.
NEVER Vac more than 1/3 the gravel at a time as that is wear your bacteria live., to speed the establishment of a good bacterial colony just take the filter sponges out of a powerfilter and squeeze the sludge from them onto the gravel over a working plate and stir it in a little, this will save days.
Ugfs are a bit slower than cannisters and powerfilters so alone won't support as large a bioload as they will, and most keepers over load the tanks, but they do work well if you let them and make a great suplemental filter that will give you time to fix a problem if the other filter fails. If you have a powerhead fail just shove an air tube down that uplift tube until you get it fixed.
One note, once established ugfs cannot be just turned off and left, within a few days the aerobic bacteria will crash and your tank will turn anerobic. Not good!
Most people who have problems with ugfs just fiddle with them too much, I knew someone who was very disatisfied with the performance of his set up and it turned out he was doing a complete gravel vac every 2 weeks, that would be like replacing the bioballs in a cannister with new ones every week. all that is left is some mechanical filtration. Once running they are pretty low maintainance.
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