Some will not raise the rents of good tenants preferring to supplement their rent in the belief keeping them is preferred over charging full market rent with the risk they will move.

Stay under market if you have a undesirable property otherwise why would you supplement your tenants rent.

It can be supplemental but is not necessary.  

Some grew up in real estate, some worked in construction which translated quite well to rehabbing, others came from a finance background and can feel comfortable working the numbers.I'd say hedge your bets and find a way to add/supplement what your currently doing with real estate.

I have some more questions which require some help to supplement my research.

I have several older tenants probably paying half their monthly 'fixed' income to rent in a safe/secure area & they need to work part-time to supplement their lifestyle.http://www.wral.com/more-americans-spending-at-lea...

Does anyone know when a carbon monoxide detector is needed to supplement a smoke detector?

Please refer to HUD's website to read more about the Fair Housing Act.As @Ned Carey said, there are some additional jurisdictional-specific protected classes that some States and Counties decided to enforce to supplement the national Fair Housing Act, one of which is protection on the basis of income source (aka vouchers). 

Speak to a professional and/or supplement the a/c with an in-room a/c or window unit.

They’re questions of supplemental income and do not go against fair housing laws.