For the future, is there any way to prevent this from happening?

I don't want sexism to prevent me from bettering my life, but this is really dissuading me from continuing.

Your current situation prevents that.I'm with @Chris Svendsen.

Obviously, I am going to authorize an after-hours call to a plumber if there is a leak that results in water accumulating  under the bathroom sink (preventing property damage is a no-brainer); however, if a toilet is clogged, the water heater stopped working, electric power is no longer coming into a section of the house, then I dont want to pay the premium to have the tradesperson come after-hours (I dont feel like paying a premium for the convenience to my renter) when the call can be made the following day or on Monday if the issue occurs on Sunday morning.  

The 2nd negative can be prevented, if you buy in a community that contributes to their reserves, and has a fully funded reserve schedule.The positives are: you have less to manage.

We want to do it the right way the first time so we do not compromise our relationship and prevent any unforeseen events.

Free Advice: Create two LLC's one for flipping and one for your passive as this will prevent domino effect if one aspect fails and make sure you setup proper firewalls to prevent commingling funds. 

My question is, are there any rules preventing me from working exclusively with investors?

It's better to prevent any "targeting" in the first place! 

It can also be used in a wall near pipes to keep them from freezing if freezing is a problem, but it will provide heat that keeps the building warm and will prevent freezing.