I worked at FSI for a short while as a sim instructor. Pay was $70K salary and the place was nice. OTOH, being salary they sure got their money's worth from the staff, with long days and lots of them. FAR 141 only says 8 hours max a day with a client. There is more to it, paperwork and prep, plus your own training, so a 10 hour day, sometimes 9 or more days in a row was the norm.
Airline sim work can be non seniority list like jetBlue U in MCO or from retired CAs at some other places. In the airline world a medical is required IF you will be doing PCs and type ratings. If just instructing no medical is required, and, as I found out for my GV ride, not required for a type ride under FAR 61!
Think long a hard about the career path, some love it, some will find it not so grand. Once you've been in a year you will be considered non current for flying jobs as well.