Technicly yes, in actual battle like the how the girls have the option to change no, because the npc/monsters are AI controled, meaning they kinda just do whatever they please...but with a helping slant.

Things I have learned on my own through trial and error-anyone else who has played this who can read and understand everythng np can help sort this out but...

1- They lvl up just like the girls do, or they are including on the ending battle screen with the xp just like them.

2- You can give them any garment grid/result plate AND choose to give them any ds you like, excluding the unique special ones that each girl has ( flora, machine etc ) including the two new DS.

3- It would seem that some items cannot be fed or equiped on them, I tried giving Brother the Muscle belt and he simply cannot put it on, many other items can easily be equiped though and...they seem to benefit from the stats of them just like anyone else.

4- I am not too sure if they gain AP or not, if so I have no idea if these abilities are hidden and randomly used by the npc in battle or if you can choose to have them do a set pattern of them, to go into more detail...it seems that you CAN give them auto abilities, through equiping items AND by feeding them certain consumable items.

I was able to gather enough from the 3 options for feeding them that certain items switch around or let them learn new abilities, this would seem right as npc's are blacked out for choosing the abilities to learn normaly, I just dont understand all the menus so I am still learning myself.

My guess is rather than gaining AP you use gil and items to beef them up to learn that stuff...

5-They benefit from the same stat changes that GG's give, for example brother was randomly buffing them with protect etc with the one grid I was using.

6- You can manualy change their DS in the static menu after battles or change their grid just like anyone else including using their already learned whm abilites, so they can act as a healer in case a save sphere is not around but they consume mp using these at the same cost as anyone else.

7- It would seem there is no limit other than the stats maxing themselves or how much gil you have as to how much you can feed them, for example I believe it costs 38 ( yes! ) potions to raise 1 ( godamn ONE! ) hp on them, or from playing around more, 3 phoenx downs to raise hp by 1 also.

Thats about all I can give right now on info for npc party members.