Hello Graham - there's no such question as a stupid question - only the question not asked ;-)
This is a controversial topic. Most answers you will most likely get, is NO. You cannot OVER-SKIM. BUT, I had an interesting conversation a few times with some reefers.
The thing is - sometimes if a skimmer is REALLY big, you sometimes take things out of the water that your corals would prefer to "eat". Then, it ends up that you have to doze/feed the stuff that was originally in the water in the first place.
Look - nearly everyone on this site knows - I SCUBA dive - and I have NEVER seen any true natural reef, that is as clear as people keep their tank water (especially the people who have HUGE skimmers, and mostly keep SPS corals)....
Personally - I would rather opt (for my own tank/s that is) to share the filtration in my system with a HUGE DSB (deep sand bed), and a reasonably sized skimmer....
BUT that's me, hey.... My personal opinion of the skimmer is: that it is a failsafe mechanism that is there to quickly remove crap from the water for in case something might go wrong.... As a skimmer quickly removes crap (DOC's, POC's, ammonia, possibly organic phosphates, etc) from the water, it can save your life stock for example if you have a fish or coral die on you.
BUT - a DSB works a lot better, together with a algae scrubber (natural filtration methods), but slower.....
As I said - wait for the others to comment - they could quite possibly have different opinions to me ;-)