As long as you keep up with your water changes I don't believe you can over skim. Some people believe that you can deplete all the trace elements from the water but as long as you keep doing water changes I don't see why that should be the case. I ran a DIY
6' countercurrent skimmer in my 29 gallon for months with no ill effect. as for a good small skimmer I have a berlin airlift 60 that skims like a champ. some people say they have to adjust like crazy all the time to fine tune it but that is not the case for me I just set and forget. I run it with a whisper air pump from walmart and a limewood airstone. People like their expensive skimmers with good reason, they are very good but in my experience my $30 skimmer performs quite satisfactorily.