With my duplex that has a garage, I offer the resident a reduced rate (but it's still supplemental income for the property) but if not I'd rent them out separately given that it makes sense for the area (i.e. might not work in an extremely rural area). 

Now I would like to complement the idea of moving there with a rental property that can supplement my income.  

Hi all,I'm interested in acquiring a small (~10) portfolio of SFH rentals to supplement my income in ~15-20 years.

I also own a cabin up in the mountains north of Phoenix and am considering doing short-term AirBNB rentals with it.My goals are to build up a small portfolio of multi-family units in the Phoenix area to supplement my income.

Best to identify exactly what you are hoping to achieve - supplement or replace income, how much, etc. 

Over the years I have always supplemented my income even when I made a very low day job salary at a full time job.

This will avoid the risk of losing a tenant over a rent increase and regardless of how good your tenant is it is poor business practice to supplement a tenants rent.It is not good business to not raise the rent as not only are you supplementing a tenants rent you are also devaluing the resale of your property.Research your local rental market adds and put your rent at market for the size and location of your property.

In the long term I want to continue to buy and hold rental properties to supplement my earned income.

What you are asking is should I be a real estate  speculator.Why do you want to be a landlord, do you know your state landlord tenant codes, will you have a property manager or manage yourself, do you have the skills to deal with tenants and evict, are you OK with tenants damaging the property, are you adverse to risk, will the property have positive cash flow or will you supplement your tenants rent monthly, do you even know what market rents are, is it your desire to own and operate a business, do you understand that equity in a rental will decrease the cash flow and possibly turn the investment into a liability, are you only doing this to speculate on appreciation, when the market turns in the future will you be ok losing money, will you hold long term when the market drops or take your losses and run ????????

We are planning to buy a few buy and holds to supplement our income over the next few years.