Yeah I have a lot of similar types right now because not all of the weapon types are available from merchants right now - Ashe/Basch/Vaan have been going the melee route, Fran/Penelo have been going the magic route and Balthier is sort of hybrid between both right now. Although I've been sending Ashe and Vaan into Green/Arcane too to pickup Decoy and Protect etc.

I figure that way once more weapons open up (i.e. vendors actually start selling Katanas and whatnot) that all the attack damage/+strength when HP is full etc will still be useful when turning someone like Basch from Sword/Shield into a 2h'er melee guy.

I've been generally trying to keep everyone around a similar level because sometimes I do end up in situations where I haven't been able to buy a certain spell (Vox for example) and end up with an entirely silenced party so I have to switch people around.

Hopefully once more stuff unlocks I can specialise people a bit more. What I'm sort of planning on doing is:

Ashe - Sword/Shield - uses Decoy to keep stuff on her and casts Protect/Shell on herself
Balthier or Basch - As 2h melee damage, probably spec'ed to throw Cures/Cleanses around as needed
Penelo - Black Magic and turns to support if the party starts dropping really low

Then having Fran as a 2nd Penelo and Vaan as a second Ashe

(This is also the most thought I've ever put into an FF game - usually I just wing it and go with the characters I like the appearance of and throw tactics to the wind!)