I'm in the same sort of situation as you and this what feels different from a playing (and winning
) two small P:M tournaments:
The biggest wtf was his throws. They seem to launch at different angles. The chaingrab on Fox feels so incredibly different it almost lost me a tournament. Whatever the chaingrab on Wolf is, it's probably worth knowing, as well.
Dthrow seems to not launch as far behind Marth. Dthrow Fsmash seemed to hit a lot more often. It's possible that my opponents' DI choices were poor, but no DI choices would result in the weird angles I was seeing.
Tippers seemed easier to get, to me. I was getting pivot tipper F-smashes all the time. The different look of the game might screw you up, though. I was also getting lots of tipper fair and utilt strings and things seemed to combo together more easily, and I'm not really sure why.
I don't really know P:M matchups at all, but from what I gathered in the past, Diddy and Roy can be upthrow utilted at low percentages, hilariously enough (I know this because Roy can be uthrow utilted in Melee from 0% and Diddy has a similar fall speed and weight).
But the general strategy I've been running for Marth to fall back on in the absence of matchup knowledge, which you might not be used to because you're used to fighting spacies in Melee, is to approach with WD dtilt, dash JC grab, and shield-stopped fair/tomahawk mixup (but mainly spam that dtilt) and punish their attempts to get around that. Dtilt is not easy to punish without some top tier speed if you tilt forward and shield or dash away, and the same goes for fade-away fairs/nairs. Their options are usually to try jumping at you, which Marth can usually just jab/side-B them out of, or dash away from.
If they outrange Marth, their careful spacing needed to beat out Marth certainly won't be safe to well-spaced dash dancing and punishing.
Projectiles? Shield and WD out, and in general stay on the offensive.
Don't know how to kill anyone that you can't edgeguard or kill from combo to Fsmash? Never forget about side-B->utilt and up-B, and dair as a starter.
From grabs either be good at tech chasing and fthrow/backthrow (Marth's tech chase is underrated imo) or upthrow them and don't let them down.
It's really hard to deal with this strategy. But by this theory, your tough matchups will be where:
-You can't combo them from throws
-You can't trap them easily from below (this is usually where you need to tech chase them)
-You can't tech chase them
-You can't edgeguard them (you probably won't know how anyway)
-They have great ways around dtilt and fair