'ABEC' is merely a standard which indicates how much friction the bearing generates.
For example, spin an ABEC 7 bearing and it will keep on spinning for ages. Spin an ABEC 1 bearing, it will only do a few turns.
With skateboards, ABEC 4 or 5 bearings are usually used... slower bearings are more stable for beginners, faster bearings make the board more likely to zip out from under your feet, and put you on your arse.
For the Shift Bearings DIY hack, any ABEC rating will do, it doesn't matter in the slightest. Get the cheapest ones.