This situation happens to me all the time in the games I play.

50/100 5-6 players. I with M of 6-10 in BB. 1-2 medium/big stake limps in middle/late position, they do it EVERY HAND. I got rags I check. Flop doesn't hit me. I check. The limpers bets 1bb no matter what. I can't tell if they have anything.

If I bet 1bb first they will definitely call. I can bet 3bb on the flop or turn to chase them off but that is dangerous when I am out of position with nothing and short stake.

Is there a way to defend this strategy?
Wait for Ax, pairs, face cards to push pf?
Problem is 2-3 players behind me including sb, and sometimes the big stake is crazy.

Or just fit and fold, 4 players pot odds justifiable?

Your rags will eventually hit and thats when you make your move. Until then you have to let them have at it. When you can't bluff people, you have to have the hand.

With a really low M (below 5), if you read these players for weak hands, push back all in PF. They'll likely fold some % of the time, and the rest of the time you won't be as bad off as you think (run 43 against AK in pokerstove and some hands of the like). If you're that low, then the cards don't matter, and with 3-4 limpers, you're likely getting your odds even if they call (unfortunately, so are they).

With an M of 6-10, most of the time I'd just check PF and fold if the flop misses me. Once in a while, I reraise all in to steal the limps.