Many things i'm confused about NLHE 6max

[soko]

Do you ever open limp?

How strong of a hand do you need OOP to bet after the flop (2-3 opponents)? Do you check vunerable hands and not protect your hand simply because you might be behind (weak kickers, medium pocket pairs on rag boards w/ maybe 1 overcard, etc)?

Is there any reason to complete SB with marginal hands with early limpers if you have to play out of position, the best you can really hope for is a hand strong enough to win the preflop calls like 2 pair (1 in 50).

Is it too tight to never play any aces (suited or not) lower than AJ and only from late position? Fold AJo UTG?

I'm sure I will think of more.