the TBS Powercube states that by default the flight mode in Cleanflight is set to ACRO but it also states that I can assign a 3 position switch on my Remote Control. Does that mean I can set it up in Self-Leveling Beginner Mode, Self-Leveling Sport Mode and full on Acro Mode? 

I want to be able to start off slow and progress into Acro when I first start. I'm reading that correctly?

That is correct you can. but I highly recommend learning to fly rather than learning to hover! Just use stable modes when absolutely necessary, the quicker you just use acro mode the better you will fly and much greater will be your experience!

I suggest that you start an Acro mode . Because if you learn to fly in stabilize mode it will be a lot harder to fly an Acro mode. When I first started I started in stabilize mode. When I went to Acro I will never go back

I think that using self level mode just long enough to get comfortable seeing through your quad's POV is a good thing. It can be over whelming just getting used to how to judge your surroundings when you may not even have your rates and expo set up yet. You also need to learn to forget your body first and be the quad. In the modes tab of clean flight enable horizon mode, assign it to an AUX switch. You'll have to move the two sliders so that the switch lands between them when in the desired position. Don't fly self level to long cause everyone is right in saying once your go acro you'll never go back.