Hmmmmm, it really depends on a few factors one of the biggest ones, what sort of condition paint are you going to be working on?
The more damaged the paint (i.e. more swirls, etc) the more cut you're going to need from a pad.
That being said, I know that Mike and many others always say go with the least aggressive option first and then if it's not enough when you do a test section, step up to a more aggressive pad and/ or product.
If you only have some minor swirling on your daily driver, then you'll probably want to go with a mild cut pad.
If your dealing with massive defects, then a yellow CCS pad may be in order, but I've heard a couple of people on this forum recommend using a wool pad if you have to use a yellow pad. I'm not 100% sure on this so if someone has more experience with this, please please chime in.
For what I'd suggest, you will probably want to get some orange pads for removing the majority of swirls, then for finishing it off a bit more, some white and gray pads.
I don't have so much experience with the different colours of pad, so hopefully someone else with more experience will chime in here.
I'm also going to be placing an order for a Flex soonish and will be interested to hear the recommendations from other members.
Good luck,
Matt