Standard instructor pay in a public gym can be pretty low. the job is mostly cleaning and customer service. As you will know from your studies, the level and type of advice you’ll be qualified to give, is pretty standardised.
Some gyms have a staged personnel structure, starting with unpaid or expenses-only ‘motivators’ (gym members who are available to advise others), and rising in responsibility to managerial positions, which can be quite well paid. Often, these positions will include the responsibility of starting a new branch; you can be ‘parachuted’ into the new location to recruit its staff and oversee the fit-out.
In between the motivators and managers are usually several layers of, basically, sales jobs (you sell memberships, and make sure existing members re-enrol each year), often paid the same as an instructor, but with commission on top.
It is a very competitive field; many more people train for qualifications each year, than there are jobs available. This keeps the salaries low.
Working out on ‘company time’ is normally regarded as an absolute no-no. You are a ‘front-line’ representative of the organisation, and meeting the public while you are covered in sweat is extremely bad manners. Although muscular physiques are expected, in order to present an ‘image’, the staff are still expected to be well-presented and tidy. This is not always adhered to, but you will find that the general standard of running in a particular gym is also reflected in the staff attitude. If you want to do well, you need to work in a well-run gym, where you will not be allowed (or have time) to sweat. Quiet gyms, and badly run ones, are the ones that close first or lay off staff.