Can you trim rim tape once installed? I installed my rim tape then noticed was too wide so I trimmed it with a razor knife. Would this have caused any damage to the rim bead?
Probably not, depends how careful you are and if they are carbon rims.

I always do a 3cm test to get the best width, then take my time and gracefully cut the width of the entire roll with a cutter. 
Now you have a entire roll of the correct width and you just apply what you need.