Those skimmers are designed to be turned off and on. In otherwords, you decide when it skims the surface and when it uses a inlet for the intake under the water. So basically, when the scum becomes too much for you, you turn on the skimmer portion.

 

A friend of mine has used the Eheim version and finds it a pain to adjust all of the time. Another option is to run an airstone at night to agitate the surface. You can also adjust the outlet of the canister filter to produce just the slightest surface agitation at all times, though this gets tricky if you have fast evaporation rates.

 

i use one and it helps a ton with keeping the surface clean.
how much co2 does it outgas? i can't help you with that one.
but the water surface never looked better.

 

I inject C02 and use surface skimmers, they don't create enough surface agitation to lose any C02.

I have one made by Ehiem, which is a pain in the butt. Small fish can also be sucked down it, know this from experience. The other one is the Fluval brand which works very nicely. Only needs adjusting if the water levels drops from evaporation.

 

I have the Aquarium Surface Skimmer from drsfostersmith.com and I love it. I have used it for several months now with DIY CO2 on my 75gallon and I can't say I noticed any change in CO2 after I installed it. SnyperP was right in saying you can turn off the skimmer part if you want, or you can just dial it down a little if your concerned about it. I just make mine go low enough that its taking water from the surface but not making any slurping sound... no big deal. I never turn off the surface skimming part though, it runs 24/7. It was probably about the best $10 I've spent on the tank, equipment wise.