Changing initial party at start(2 soldiers and Terra)

Depends what exactly you want to do. If you just want to make one of them a different guest character, that's pretty easy.

This is the part of the event script that sets up Wedge.

Code:

CA/5E49: 7F    Change character $0E's name to $20 (WEDGE )
CA/5E4C: 40    Assign properties $20 to character $0E (Actor in stot 14)
CA/5E4F: 3D    Create object $0E
CA/5E51: 3F    Assign character $0E (Actor in stot 14) to party 1
CA/5E54: 37    Assign graphics $0E to object $0E (Actor in stot 14)
CA/5E57: 43    Assign palette $01 to character $0E (Actor in stot 14)

If you want to, e.g., make Wedge look like Kefka, you just need to hex-edit it to use name $29, graphics $15 and palette $03. Then you have a character named KEFKA that looks like Kefka but with Wedge's stats. Change the properties to $29 as well and it'll have Kefka's stats from the Imperial Camp.
If you want to make it an actual party member, that gets a little weird because they'll be overwritten by the moogles later. But if you don't care about extreme cases where you get that back character at a lower level than when they left because you leveled up a lot in Narshe and then get as little XP as possible after, you can just fake it the same way.

If you want to change the size of the starting party, you'll also have to edit some of the other events in town and the in-battle events with Whelk and Tritoch.

I got you there, this character in question isn't going to be permanent(in fact, this character is the main bad guy) the girl character I have replacing Kekfa will leave after the boss fight, so it will be a guest character. My only concern is that I don't want to have a bunch of soldier sprites looking like the girl that I have replacing Kefka later in the game.