At 263gph the turnover rate is a little on the low side IMO. I try to have 10x the turn overate or more but I tend to over filter. That being said, you probably can do just the 2217 if you keep up with large weekly water changes. As far as the fear of loosing beneifical bacteria, if you clean the filter with tank water only and not "clean" the bio media, you really shouldn't loose much. Keep in mind the filter isn't the only place the bacteria lives. It's in the substrate and on decorations and rocks etc... The down side of just the canister, is that if it breaks (which they do) then you don't have a filter. With 2 you always have at least one filter running if one breaks.
I have a fluval 305 and an Emperor 400 HOB on my 55g. The HOB is a cheap way to get alot of water turnover (around $40 online) and it keeps the surface of the water disrupted thus oxygenating the water