I prefer to use multiplayer parties so that I can create each character, but in order to do some of the subquests I need the NPC's, is there anyway to get back one of my created characters after I've subbed them out of the party, or are they gone forever?

Gone, Poof!

The game doesn't know what dialogue to give them ("Do you really want me to leave the party?"), so it doesn't give them any. They kicked out, they gone.

You might be able to avoid this by sticking the party member into a boring building alone, and booting them from the outside, but I doubt it.

You could always plan a 5-person Multiplayer party....

Hiya im new to this multiplayer option and how would you create your own party?

Just start a Multiplayer Game instead of Single-Player. I think you have to choose the "Serial" connection option, at least that's what I did. Then you're taken to a screen where you can assign control of the 6 slots of your party to the people playing the Multiplayer game with you--meaning you can assign control of all 6 to yourself, and then roll your own character in each of the 6 slots.

Also, after creating your custom party, save the game (in multiplayer) and copy the saved game into a singleplayer save game folder. Since now, you can play your custom party in a singleplayer session and avoid the multiplayer overhead.