squirtle has guaranteed jab out of ground breaks on almost all characters. some exceptions are luigi and ike. this is EXCELLENT and often better than a throw at mid to "before dthrow kill" percents because the pummels + three jabs will refresh your stale moves very well. squirtle can hydrograb to the edge and then fall off the stage as the opponent is released to put you in a good position to gimp. lots of different applications with this, try your own. it's pretty promising, but you can get gimped yourself if you aren't careful. at low percents, you probably want to stick to uthrow -> utilts. uthrow is the most damaging and it's very easy to get those utilts up. bait their air dodge/attack and punish. fthrow is kind of cool when you are fatigued because it can pseudo chaingrab some opponents at 0%. never ever use dthrow unless you plan on killing/switching with it. otherwise use common sense. fthrow and bthrow when it would put them off stage and set up a gimp. stuff like that.
ivysaur dthrow and uthrow are the best for setups. at 0, i think bullet seed is inescapable if they don't horizontally di the uthrow. particulary against fatass characters. baited uair/usmash work well out of both, and vine whip can work out of dthrow...if you are lucky. fthrow i dislike, i don't find anything special about it. i doubt it can "chaingrab" like squirtle's when fatigued but its possible. the setups from dthrow/uthrow are going to almost always be better anyway. at killing percents, uthrow and bthrow both kill at similar ranges (150+ fresh), it's just a matter of which way you think is going to expect and di, and then use the other one so the di works against them. ideally. also ivysaur can release grab squirtle to the end of the stage, lol. maybe meta knight but it's doubtful. not like you could follow up with anything anyway...
charizard is pretty similar to squirtle in his usage of throws. at 0, he can link a bthrow into an fthrow against all or almost all of the cast. good for getting them offstage right away. uthrow sets up into beautiful uair juggle traps, and utilt/usmash help in that regard as well. dthrow really does not kill well at all if they di up, but it's probably your best shot. that upward di can help you and get a uthrow kill, but its unlikely.
hope i helped.