Is there any standard guidelines for balancing hop bitterness with malty sweetness for the different styles?
Loaded question I know, but I made an IPA around 55IBU but it only had carapils as the steeping grains and it's almost overly bitter for me, whereas my previous IPA using steeped crystal malt was much more enjoyable depsite the same bitterness units.
You could leave it longer, and let the bitterness mellow a little. 55 IBU isn't that bad, but some hops make a harsher bitterness than others.
In terms of balance, try Ray Daniel's book "Designing great beers". He has a BUGU ratio thing, where the bitterness units are expressed as a number, so a 40IBU beer in a 50 gravity beer would have a BUGU ratio of 0.8, and in each of the sections (Bitters, lagers etc), he uses the BUGU ratio to help.