A few points:
Just to reaffirm, the gas pockets are not an issue unless you go deep sand, 3" or more.
If your filter intake is off the bottom, maybe midway up the tank, sand will not be a problem UNLESS you don't rinse it really well AND turn it off when doing water work (changes, planting, moving substrate for any reason). Even then, with a HOB it would serve you well to take the filter off and clean out the impeller area as the sand can settle in there and cause wear and tear on the workings. Foam on the intake will help.
55 gallon, I would consider going to a canister if you've the budget, they don't need to be expensive and the pump is on the clean side of the flow.
Add to your corys, they should be 6 or more. In a 55 I would say 10 is a good start for them. This will help if you go with a semi aggressive fish too.
Consider plants. They are great for the fish. You would need to make sure that your light was adequate firat though.