I'm pretty new to Marth and Melee in general, but I may have found a chain grab on Peach while fighting vs AI, I don't know how well this actually works against an opponent with good DI, but @ ~15% (maybe a little higher but it definitely works @ 20%+) you can chain throw forward throws, Grab > FT > Dash > Grab > repeat across the stage into an Fsmash OR Short hop Down air. I discovered this totally on accident and got excited as **** when I realized it could combo into a down air. It works especially well on FD, probably works well on Dreamland. I haven't tested on battlefield / yoshi's, FoD might cause problems depending on platform height. I'd like to know if anyone can confirm this works against good DI, otherwise Ill have to wait til next wednesday to confirm for myself. This works to a pretty high percent, and if they're near the edge, F Throw into down air probably combos at any percent. I really hope this actually works :D. Anyway hope it helps.The down air is harder to get @ lower percents, but at those percents you can at least get a solid Fsmash
Another edit >.>, anyway Ive been able to do it starting from 0%, the 10% - 15% range I'm unsure of; in recent tests I've been getting it consistently so it should be viable.
So, Down air isn't always possible depending on their DI (although you might just have to be a better marth player than me in order to always get it, seems like being able to dash > SH > Dair helps improve the hit rate, at least for me) amd F smash stops working past ~50%. But the chain grab itself works up to about 100% vs. AI where it starts to get inconsistent so presumably actual players wouldn't get caught anymore.