Diddy is sooooooo much easier to gimp than Marth. Atleast for me haha.
Depending on how you decide to space yourself, you can usually either uair, or dair his side B on reaction.
If he recovers high, he can be tough to gimp, but still juggleable, especially if he has no second jump, and juggles can often lead to hitting him off the stage with no second jump, where he has to recover from the side, or low.
I guess there are a lot of ways to gimp Diddy, but I tend, when he's recovering from the side, or low, stay above him, and between him and the ledge, and punish anything he commits to with tippered dair (the angle of a tippered dair, as opposed to the inner hit from it, is a lot better (or maybe it just has more knockback)).
Oh yeah, Rain's SL **** *****. One mixup people don't expect is doing an SL that they air dodge off the stage on SV, and then platform dropping to dair like immediately.