Changing actors involved in events and recruit locations

I would suggest loading up Zone Doctor CE and viewing the events you're looking for until you understand them. As far as changing which characters move and speak during a scene, that's a one-byte hex edit; simple to do. As far as rewriting the scenes, it just takes a keen mind and a lot of trial and error before you get it right.

My method for event editing is to rework/block a scene in Zone Doctor CE, save it to a new rom file, and then use a hex editor (Cygnus) to copy/paste the event into my main working rom. I then fill in any unused space with "FE". Most of my edits have ended up making the scene take less space though.

In general though, when you encounter a character, you need to do the following:
1. Create an object (00-0D)
1. Assign Actor attributes to the object (00-0D)
2. Assign graphics to the object
3. Assign a palette to those graphics
4. Set the name for the object
5. (optional) Invoke the name-change screen for the object.
6. Remove all negative status effects from the object
7. Set the HP to max for the object
8. Set the MP to max for the object
(Note: 6-8 are necessary because the character slots are taken up by the 11 Moogles in the opening, and those moogles may have sustained damage or received status effects)
9. Assign the object to a party.