For basic strategy, is it best to just learn what you'll be playing 90+% of the time or should you learn single deck with standard rules and add on adjustments for # of decks, rules etc.?

On counts, the first count I read about was red 7 and I was going to proceed with learning it. But here, it seems different versions of KO are recommended beginners. Is R7 significantly weaker than KO or does it have to do with the limited number of index plays highlighted in Blackbelt in BJ?

Appreciate any advice/opinions!