^^^ this. While different companies have their own policies, it has nothing to do with the regs. You can instruct as much as you want without affecting your 121 duty limits. At OO, they don't care if you instruct on the side so long as it doesn't affect your ability to do your job.

Trust me, I know this all too well from doing rental checks for airline pilots. That is why I still want to do some CFIing. Plus, I want to stay current in-case I want to do it when I am forced to retire from 121.

Are you saying there is no difference from flying jets on auto pilot for 90% of the time up in the Stratosphere compared to GA flying? You sound like one of those 10,000 hour pilots who comes in for a rental check without flying anything under 80,000lbs in over 10 years other than a pen whipped BFR from a CFI buddy and fails miserably.

Which rest rules do most Regionals fall under?

Follow your company procedures regarding outside flying and don't get yourself in a jam...

117 applies to all 121 passenger carrying operations. Only cargo ops operate under the old rules.

This. One airspace bust or other violation and your on the side GA gig just put your "real" job in jeopardy. ASRS will not protect you to the degree as your 121's ASAP. Why take the risk in the infancy of your career?

Most of us breathed a sigh of relief when our last day in a GA airplane (usually as a CFI) came around and we escaped unharmed. Why take on the additional risk?

Most airline guys I know who do GA are established mainline types who have extra cash to burn. They aren't trying to find the next better job so they don't have as much to lose.

Why would an airline pilot need a BFR?

I don't think keeping yourself out of trouble in the 91 world is really that difficult.

Just check with the specific company. And it shouldn't be that hard to keep yourself out of trouble doing ga flying.