You might even want to go with two slightly smaller Aquaclears. That way you have water movement throughout the whole tank not just on one side. It will cost slightly more but thats just my opinion. If you do I would recommend the Aquaclear 70. The AC 110 will give you 500 GPH while having two AC 70s will give you 600 GPH (300 each)
In case you don't know what GPH means its stands for Gallons Per Hour. I personally would go for the pair of AC 70's not only does if it give you what I said earlier but it also means that if you ever decide to...ohhh I don't know...say switch to a monster

LOL you won't need to buy another filter.
I'm actually switching my 55 over from a Oscar tank to a "community" tank, although it might only be stocked with Tiger Barbs. They are usually considered community fish but are slightly nippy to long finned fish like male bettas ( Siamese Fighting Fish )
I also second on getting the 75 gallon, you will be open to MUCH more choices in fish. I regret not buying a 75 instead of my 55, but live and learn. Don't make my mistake! Plus you might even get addicted and want another tank
Spencer