MC is the only one who can currently start in the close position so for everyone else, ranged combat is superior to melee except in long battles because you have to waste a turn moving in. With that being said, a pure melee strategy still works fine in pretty much all situations. It just takes a bit more work to get there. You'll want everyone to use a greatsword and that requires at least 75 strength to avoid a damage penalty and you'll also want everyone's melee skill to be high enough to be guaranteed to hit so somewhere in the 120+ range.
I would say for a pure melee party you're going to want everyone to have either warrior or brawler, preferably both. Malevolent and Crackshot also help. If you're going with an all slave party, health is going to be a potential problem since most slaves cap at less than 100 health. You could give them all stealth suits, but then MC has to solo tank everything which is literally impossible for the toughest fights unless you're ok with MC taking permanent stat losses every time he gets taken down.
In long fights melee actually out damages plasma but in shorter fights you will consistently take more damage than you would with ranged because of the need to waste a turn moving in. In long fights the increased damage makes up for that wasted turn but the first round still hurts since you won't ever get a kill that round unless MC 1 shots something.
Some good options for a pure melee party if you're not using the top tiers would be Dexter or Kelly for their Crackshot ability, plus Rebecca, the ex arena champ in Kahill, and Zenobia for their guaranteed combat traits. Some of the lower tier mercs also have warrior, brawler, or both and could make good options because of that.
