Yeah aside from Cactuar and M2K, the only Marth I lost to recently was HBK (he is underrated and very good) in a close pools set. I can play a lot of random low tiers at a decent level, but for sheik I have been using fox. Against Marth, Falco and Jiggs I usually try to stick it out with Luigi,
Looking at Luigi's troublesome matchups Falco, Marth, and Sheik, I would say a decent fox or falco is your best bet as a secondary. Falcon, Jiggs or Doc is a decent choice as well. I would suggest Falco because Marth and Falco are both very feasible matches for Luigi if you want to main the plumber. Sheik on the other hand is just gay and Falco is probably sheik'd worst matchup.