can i hook my canister filter outlet up to my protein skimmer after it takes it from my sump? And if i do a 15% water change per month, is that enough?

yes, but it would be better the other way round so the skimmer takes out the waste before the filter has to.

and yes 15% a month is okay, unlesss you get high readings of nitrate, phosphate etc

ok, how do i get the water into the skimmer...another small pump?

yes or by having the overflow from the tank going directly to skimmer is best