Playing small pairs is a balance of factors.
I guess the big factors are:
1) Are stacks big enough to justify set mining?
2) How likely am I to win the pot pre-flop?
2) How likely is the person to put a lot of chips in if you do hit a set?
3) How likely is the villain to fold post flop (if he misses, or to a steal, or to a scare card?).
Your ability to get paid off when you hit a set is vital. Your
odds of flopping a set with a pocket pair is 8.3 to 1. Often when you flop a set it misses other players, so you don't get paid off post-flop and win a small pot. This hurts your overall average return, so you need to be fairly likely to win more than 8.3-to-1 to counteract the times when they fold post flop and in order for set mining to be profitable long term. I'd say you need have a reasonable chance at winning 10 to 15 times your initial investment.
In early stages of MTT's there are plently of bad players who seem eager to overplay hands like top-pair, and invest way too much money defending their not-so-good hands. Also, people have huge stacks compared to the blinds, so you can get in cheap and these people will pay off 20x your return. So early on, I'll probably just limp with a small pair as there there are too many callers who play garbage. When they miss you win very little post-flop. If they do hit something you can overcharge them post-flop. There's no reason to invest chips to build a pot pre-flop when you can do the same thing if you hit the set.
For most of the game though, I prefer to open pre-flop with a small pair, especially in late position. This gives me my best chance at winning as people can fold pre-flop to give me the blinds, or post-flop I can c-bet and take it down as 2/3 of the time the flop doesn't help them.
To call a raise, you need to be sure that there are enough chips to justify the call because you no longer have the fold
equity.
With 25/50 blinds and a 1,500 stack and the big stack at the table opens to 150, you should probably just fold your 5/5 *unless* you have position on him and he is especially passive post flop (and if you are confident in your post flop play).
With 25/50 blinds and a 4,000 stack and a villain with 1,200 opens to 150 you should probably fold as well, as you can only win a total of 1,200 off him which isn't enough to make the play profitable in the long term.
With 25/50 blinds and a 4,000 stack and the big stack opens to 150 I am eager to call.
With 25/50 blinds and a 1,500 stack in late position or any of the blinds, and a total nit opens to 150 under the gun I call with my 5/5. In the BB or SB it only costs me 100 or 125 more to call, and in late position I have position on him post-flop. The big reason for the call though, is that a nit under the gun is substantially more likely to have a big hand that will be dominated if you hit a set, so my potential payoff can be less and still be profitable long term because when I hit it I'm much more likely to get his stack. For example, if he has K/K and the flop is 4/5/Q you are usually going to get all of his chips. If he has A/K and the flop has an A or a K you will get all of his chips. Of if he has A/K and the flop is 4/7/Q (totally missing your 5/5) he'll check and you can take the pot. There's just so many way to win.
