Two ways to do things:
1) Traditional molle attachment system. Secure, but can be a pain to get set up initially, not super customizable once it's set up.
2) Velco/buckle attachment using placards. Much easier to change things up quickly.
Example of each: Ferro Slickster (not complete yet) on the left, Grey Ghost Minimalist on the right.

Ferro with placard removed:

Anything that uses the QASM buckles and velco backing can be put on a carrier let up like the Slickster. So if you have ARs, AKs, .308s, PCC....someone makes a placard for it, and you can change out what you can put on your carrier in seconds. I have a 3 AR mag placard from Esstac on mine right now, with an additional torniquet holder. As well, you can completely remove the placard and run the PC slick with just plates and nothing attached if low profile protection is what you're after.
Both are good options. Start with buying a carrier, then get plates that fit it, accessories that work with it, and go from there. IMO.