there was some question about how to handle doc earlier in here, i believe i may be able to offer some insight.
at low %'s, doc downthrows reeeeaaally nicely. i think unless he has perfect down and away DI, you can nair him after a downthrow at low %'s.
from here, you have to act depending on DI:
1) no DI, you can either nair again or regrab (i prefer regrab, to start the combo over again. after 15 or 20%, no amount of DI away will save doc from being arieled after a dthrow)
2) if he DI's down and away, he will have to tech, i suggest throwing a stomp where you think he will tech and combo from there
3) if he DI's up, then shffl up air.
regardless of which DI he choses, if you follow up correctly, you will be able to hit with another ariel that can lead to a knee. doc is the number 1 character that i perform 0-death combos on, and 0-death (or below 20% to death) combos on doc generally happen one every 3 matches when i play against a doc in my general skill level.
i could be wrong, but i believe mario has the same fastfall and weight physics, and can be combo'd either identically or very close.
at low %'s, marth combos quite similarly to doc as well, and many of what i said applies to marth, too.
one of the best ways to gimp a marth is to dthrow a marth, SHFLL a nair in to him so he goes off the stage. jump at him and upair, then double jump up air again. unless he was at a VERY low %, he will be pushed too far to survive and edge hog.
to edge guard marth, try the light shield technique. roll backwards to the edge of the stage so you are facing away from it, but are REALLY close to falling off. light shield, and angle it down and off the stage. when marth comes to upB, he will knock you off and cause you to grab the edge, and will be edgehogged. good marths will see this and time their upB to land on the stage. if this is the case, a simple ledge hop KNEE will end that marth's stock.
luigi dies to this trick identically, except that luigi can NOT land on the stage with his upB. if a luigi needs to catch the edge with an upB and you correectly perform this trick, it is a guarenteed stock.
hope that helps.