My approach as default for SB limping is to make it in quite unbalanced way so not that much limp-3betting. Reason being that population tend to be fairly passive versus it. Which means that trapping tends to face too often checks and we ultimately lose too much value in process. I would say most effective stack depth for SB limping is one where facing 3bet shove is high risk. So e.g. 15-25bb. Lot of even fairly strong hands cant really raise-call versus shove especially with ICM premium added. Plus ICM tend to be on heavy side in 18man at least if its still 40-30-20-10 structure. So limping is not necessarily that polarized as there is such smallish portion of hand to raise-call in first place.
One common type of hand class to limp-shove is small to medium pockets with like 18-30bb stack or so. Reason being that BB should defend wide versus small raise having position and hand like 55 suck playability wise. Also we dont love to raise-call and have no fe. So by limping we keep pot small or get chance to limp-shove and go for fe and reasonable equity versus raise-call range. Well facing shove can be issue though as call might be -ev because of ICM.
There are lot of approaches in SB limping. Versus solid opponent it can even be reasonable to limp most of your range with 20+bb as fe with raise theory wise is so low. But then again lot of players fold too much which make steal more effective. Often versus reggy players you want to be careful of having protection especially with those 10-15bb short stacks as otherwise they might print ton of money with wide shoves. Same of course apply with deeper stacks as putting pressure on our limp range is very effective if we are not prepared for it.
I have not really studied any gto approach but as always ok approach is to think ahead what you want to happen. E.g. you have QJs with different stack depths:
- 15bb or less: Probably shove unless ICM is so low you can limp-call versus shove.
-15-25bb: Likely limp or shove as raise-folding would be disaster. Also using great postflop playability hand to shove 20bb is bit meh.
-25-35bb: Facing shove less likely and we can also raise-call with good playability versus small 3bet so raise is more reasonable.
35+bb: No issue in calling versus 3bet so usually just raising.
Also as default having small raise range might be sort of bad when we are lets say 15bb or so deep as shove still have decent risk:reward and trying to have balanced ranges for limping, raising and shoving would be quite challenging. Then again know your opponent as if BB is overfolding making that 2.5bb raise can be very effective.
Roughly equal stacks 
