I think depends on the table and how often you are getting 3b opening from this position. Usually 55 is the bottom for me. Limping results in bloated poats oop and this will never be good with a hand weak and with so little board coverage as pocket pairs.

I find 55/66 being my cutoff as well, would you say fold or raise here the best two options based on table dynamics?

I'm think limping looks just like a small pair or SC and invites a raise. If you are going to open I think a min raise is best.  Sometimes you can open UTG and if you are tag get folds all the way around either pre or to a flop cbet

Open fold 99 or 10s from MP? Never

Yah I'm never open limping as a general rule with these type of hands. Obviously the later position we are in the more I like opening. 55/66 is a good cutoff from very early position when the stacks are getting shorter at the rest of the table and we know people are going to be pretty aggressive 3betting/4betting. Generally at the small stakes people aren't light 3betting so its even more reason to tighten up a bit.

I think to answer your question about what to do against a 3bet you need to range each opponent based on our knowledge of what they've done. Its a tough answer to give without knowing the opponents but yes 99 is too big. Its not horrible being OOP when called we just have to focus on how to play the hand profitable based on the board texture, opponents stack size and tendancies.