When I hit the bubble, and everyone is busting out/rebuying, I tend to target the guys looking to double their chips.
Lets say I have 50k chips, on the button, everyone else folded, and the guy next to me in the cut off shoves 6k all-in. I'm always inclined to call with marginal hands. because to me, it's 50/50, if I lose I'm not in terrible shape, and he's probably shoving with ANY highcards.
Is this a bad habit? .. if so, why?
It really depends on how much it's costing you in terms of BBs. If you have a significant amount of BBs, say 83 if blinds are 300/600, then you can call with a bit of a wider range but not necessarily any 2 cards. It's probably still a good idea to fold the weakest of holdings. If villain is shoving high cards, you only have 40%
equity if you aren't sharing any cards like 25o vs QJo. However, given their shove range vs calling a weak ATC hand, your equity is much lower.
Another thing to consider is the blind behind you in this situation, their stack sizes, and what they might do. If they're short-mid and capable of shoving to isolate, then you're better off folding off your weak hands and 3betting with your strong hands. You want to avoid awkward situations where you call off a 6K all-in shove with a hand like 39o only to face a reshove behind you for an extra 8K. It's hard to fold for 8K more when you have 6K invested, but you're putting in 15K or 30% of your stack with a very weak hand.
Also, is it better to target this chip leader of the table? .. I find it hard to bluff chip leader, and i'm always getting bullied around folding medium-sized pots. Any suggestions?
It's actually better to attack medium sized stacks.
Big stacks are capable of calling with a wider range of hands, can splash around in pots without risk of busting, and put tons of pressure on you. I'd only play with strong hands against them or set traps with small pairs/suited connectors and hope to hit a monster flop.
Small stacks are in shove/fold mode so you can make small raises against them to pick up blinds with a wide range, but usually to fold vs a 3bet shove from them unless you have a decent hand. If they shove, then you should call with a pretty wide range but not necessarily a huge one. Unless villain is a maniac, they're still shoving with a lot of pairs, high suited connectors, broadway hands, and Ax. So, your call range should be able to beat their range or at least flip with their range. Again, depending on stack size and how much of your stack is at risk. Of course, if they're shoving only 1 BB and you close action, you can call with anything. It's different if they're shoving 10 BBs or 15 BBs.
Similar sized stacks are better to attack because they a) can't put as much pressure as the big stacks and b) usually wont splash in around a lot of pots in fear of getting busted out before the short stacks. Of course, this really depends on the player. If they're ones who has tighten up and are folding a lot to make the money, then attack them more. Don't be afraid to raise against them a lot or 3bet light vs them as they usually won't play back with weak hands because of the fear of busting out. And if they do, you can just fold, then attack again when the opportunity comes.