For Marth: There's two things that you always should keep in mind:
1) Luigi will destroy you, if they get a solid opening, but Marth is really good at avoiding just that. So when you see a "hmm, I could go for option A, it's about 50-50 whether i get a combo starter or be punished"-situation, don't go for that, but look for option B, which might just marginally increase your positional advantage, but is perfectly safe
2) Luigi is relying on his ground game; he doesn't want to be in the air. This one has some more implications than the first... a) His approach is going to start with a wavedash, they aren't going to SHFFL at you. Since we're Marth, we know that tells us that we really need to be mindful about how we jump, since our air to ground game doesn't quite cut it in most situations. On the other hand, downtilt is really good to stop just that. Against Luigi you get the added benefit that even sourspot dtilts are quite safe on shield, as his low traction often makes him slide away so far that he can't get a grab. Be mindful that it can be punished, so don't be completely obvious about it. b) the wish of luigi to be on the ground is also easy to implement in your punish game: just get him above you. You need to be a little careful, because Luigi's nair has a lot of downward priority (think falco / link dair), is really quick (frame 3) and will quickly reverse the situation on you. You can still outrange it from below (our utilt / uair are pretty stupid), but often can just wait for him to throw it out (they like doing that) and then punish it afterwards, because of his low aerial mobility (or use the lag he'll be in)
To put it in the specific aspects you asked about (with some additional stuff)
neutral: lots of dtilt. Dash dance > grab can also be nice, as usual. Only jump when you are sure he can't/won't WD below you and upsmash/uptilt (or throws off your spacing, shields, and punishes from there). If they are coming from above, uptilt / fair (for the more diagonal approaches) can stop pretty much anything they do.
comboing: conservatively. Be sure what you do will work, often you can't true combo. Positional advantage is really huge here, so that it might just as well be a true combo. Random tidbit: If you can get them to nair near the ground and move out of it's range, you can often get an easy fsmash tipper.
throws: up, always up :D
edgeguarding: we haven't really touched upon this yet. Don't listen to the people who say that edgeguarding Luigi is easy, they are spewing nonsense stemming from never having played a Luigi with a decent recovery. If Luigi makes it near the ledge while still having his DJ and charged down-b, their plethora of options, timings to choose from due to being floaty as hell) and their ability to grab the ledge from really far away makes it impossible to get a guaranteed edgeguard. So well, just try nd hard read something (ledgehop dair is pretty good... gives you a shot at killing them, and is unpunishable if you miss) and don't be upset when they don't get hit by it.
For secondaries: Falco does win, but messing up gets you punished even harder than you would be with Marth. I think Sheik is pretty easy to play vs Luigi, especially in ntsc (Australia uses that version, right? :D). Dthrow leads to one or more up airs to build percents, and upair/fair to finish him off. SilentSwag plays the matchup exceptionally well, imho.
