In the airline industry you are restricted to 900hours per year, does instructing eat into this 900 hours as it is for reimbursment or does it not restrict your airline hours? Are the airlines happy for you to have this side job?
Yes, 900 hours limit. (As my lot send us around in taxis all over the place, there is very little chance of getting near 900hours, more like 500-600)
Yes, paid instruction counts for flight time limitations. (Unpaid beow 1600kg doesn't, I think).
Yes, airlines do allow it, usually just ask the fleet manager or chief pilot for permission.