Sorry if I'm a bit late in posting this, but there is a way to do what you're asking.
There is a save editor called FF7SaveEdit thats in German. I don't know who the author is because I downloaded it a long time ago. I may or may not have gotten it from these forums but I can't remember. I did a search with the name of the program to find a post about it but couldn't find anything. You can change it to English after you open it.
Move the save file (SaveXX.ff7) that contains the save you wish to edit to the same directory as the FF7SaveEdit program.
Basically, open it, load your save by double clicking on the one you want to mess around with and that'll open an easy-to-use editing menu and summary of the save you've just opened (Gil, Playtime, etc.)
Click on "Sprache" then "Englisch" at the top to change to English.
Then click on "Characters". In the "Miscellaneous" Section on the upper right, you'll see the name of the Character, which you can change if you want. Directly underneath are two checkboxes.
One is "In the PHS Visible" (You can revive Aerith this way)
Directly below this is "Changeable". The only one that isn't ticked is Cloud's. Tick it to be able to switch him in and out like every other party member.
The only thing I've noticed when I do this is that some other party members don't show up in some situations. Again, sorry for the lack of info (haven't played the game in about a year), but I remember certain areas where you could control each and every character as their own model. Cid and Tifa already have paths so they show up about 95% of the time. If you had a party of say, Vincent, Yuffie and Nanaki, there's no way to tell which model will show up if any at all. If your previous party contained Tifa then Tifa would appear on the world map, or Cid if Cid or Cloud if Cloud, but in some towns etc, the ACTUAL characters show up because they have appeared there at some point in game. I'm pretty sure you can control Nanaki in Kalm from what I remember(among other places).
Sometimes no models show up at all, in which case you control an invisible model. I just guide it back out of wherever it was and change my party.
Sorry for the long post on a topic thats a little old, or if you already found your answer, I was searching the forums for ways to avoid the invisible character problem. Does anyone think it would be possible to program paths for all characters on all levels rather than just Cloud, Tifa and Cid?
If you can't find the program I mentioned PM me and I'll send it to you. You can do all sorts of neat things with it like bring back the tiny bronco and buggy (though there's no point other than just to have them ^_^).
