I also use excel with shrimp and have had absolutely no problems.
Instead of rummy's or cardinals, consider neons. I find the rummy's that we have in out 75 gallon are very active and need a lot of space. I think they'd be cramped in a 5.5.
In my 5.5 I've had guppies, neons, platys, otos, corys and now, a lone betta.