I just started to set up a frontosa tank. I was wondering what would be good tank mates for them, i am also lokking for a good invert, but not going so well 

I agree, frontosa's grow to 14" which is longer than the width of your 55g they also need to be kept in groups of five or more fish. It you cannot up grade to a 180g min do not buy these fish.welll your gunna want atleast an 180 gallon when they mature.