Most of my dwellers are all lvl 50 with all special stats maxed out at 10. So what do I dress them in does it even matter at this point? I've been putting them all in formal wear hoping a little extra luck might help find better items.

Depends on their role in your vault ... for explorers I try to give everyone something with at least +1 luck (although I don't need the extra money), if you want distance then go for Endurance ... for resource workers I give them an outfit suited to the room they are working in (+5 Strength for Power, +5 Intelligence for Medical, +5 Charisma for Radio, etc)

I have found that it doesnt really matter at max level max stats it seems, at that point your caps and resource production is usually insane, as for explorers, I havent seen much difference in quality of gear from my +7 luck dwellers, honestly at that point, go with what makes them easiest to identify, if your explorers struggle to make it to the 60 hour mark for the chance at the legendary, maybe add a little endurance.

My explorers all wear uniform armor, females have soldier gear, and males have heavy/sturdy battle armor, thats again more to be uniform than the stats themselves

At max stats and level, it doesnt matter, Fashion Shelter is a go!

Luck doesnt seem to determine your itemfind. Luck seems to be strictly related to caps per minute found. Luck also doesn't relate to rushes, the total amount of skillpoints related to the room decide on the rush %.