As long as the insert fits, you can usually mix and match as much as you like. You might have to try different things though, as each insert will be specifically designed for that nappy so it would normally fit better than other ones. For example, a Flip stay dry insert is the right width and length so you might find that other inserts don't cover enough of the wrap, or slip around a bit more. For the organic Flip insert though, and the Econobum inserts, they're basically just a prefold (a flat nappy that you fold into three to make a 'pad'). If you got generic prefolds, you could probably use them in most nappies as you can fold them to whatever size you want. If I recall correctly, wonderoos have quite a distinctively shaped insert, but I think you buy the nappy with the insert anyway, so it's not like you'd buy the outer and then get cheaper inserts. As wonderoos are pockets, you can't reuse the outer before washing, so you only need one set of inserts per nappy I think.
If it's for a nappy like a Flip with a stay dry layer on top of the booster, just make sure that you don't buy a plain microfibre insert and put that directly next to LO's skin. Microfibre can irritate, so you really need something between, like a fleece liner.
Most boosters fit most nappies. I use pop-in inserts/boosters in all of my night nappies to make them last longer. As long as you cover the poppers with something they're fine - I use the night booster in Bumhuggers, the big day inserts in WNNN and the small day insert in all sorts!
Edit: Tots Bots Easy Fit are pretty much AIOs. They do come with their own booster for if you need it. You wouldn't need additional inserts unless you have a heavy wetter. Actually, the small pop-in insert is the perfect size for in the pocket.
