Losing audio using interface combo setup through Mac Audio Midi controller

[AdrianJ]

I have just built a Pro Tools 2018 system on an iMac - 3.4 GHZ 16GB men. OS- High Sierra 10.13.4.
I've combined an Apollo Twin MkII with a Focusrite Scarlett 18i20 through the Mac Midi app (because I didn't have 2500.00 for an Apollo 8 with an expander and I wanted to use a midi port to record a live drummer through a Mimic Pro)
Now, I am new to the Pro Tools world having previously used a Cubase system with a Layla card, so I am struggling my way through the I/O gauntlet while also trying to learn Pro Tools, Ableton Live, GarageBand and all the plugins at the same time, and I'm not that young anymore, so I'm sure I am the root cause of my own frustrations, but, four times I have had to delete the Pro Tools folder in the Library/Preferences/Avid folder because I lost the audio output while trying different routing extravaganzas. Twice I was using AbletonLive with rewire, but once I was trying to re-route the monitors to a fader in Apollo Console according to the UAD instructions that said it could be done, and the last time I really don't know what PT didn't like, but I lost the audio anyway, which makes it tough when you're new and don't know if what you were trying to accomplish should have worked or not! So my question is: "Is this likely the result of trying to combo the Apollo and Focusrite together or is it just because I'm new and don't know enough about what I'm doing?" If it's the latter I'll keep plugging away but if PT doesn't like the interface configuration and it just won't work then I don't want to waste too much time because I don't have a lot of that left! Any suggestions are much appreciated! Age