I just think DL64 hurts Marth's platform game too much. Also, I don't like giving Fox lots of room to roam free.
Comboing is mostly a mixture of tech skill and experience. Generally speaking, you should hit them up. As their percent gets higher, unspaced fairs and bairs will bring them back down to stage level. Ideal finisher would obviously be a tipper, but anything that puts them off the stage (which is where you want them) is good. Grabs can help extend your combos, but you won't be able to do much from them at higher percents unless you're confident in your tech chasing or you can trick their DI or something. If you grab them at around 100-120%, uthrow > nair at least puts them off the stage.
Just kill a level 1 Fox on FD for awhile.