I just answered on the Steam forum. I reproduce my answer here if other people are interested:
0) [Optionally] Configure the languages in "File -> Preferences"; the single-language mode will make loadings go faster (and the script file smaller in size),
1) Open the game's FF9_Launcher,
2) Go to the "Environment -> Fields" panel,
3) Use "Batch -> Export Field Scripts", let all the fields ticked and export them into a single file for simplicity,
4) Using a good text editor, open that script file and search/replace regex expressions using the followings:
search:
set Setting_PartyReserve = [0-9]*
replace:
set Setting_PartyReserve = 255
search:
AddParty\(([1-9][0-9]*)\)
replace:
\( AddParty\(4\) || AddParty\($1\) \)
search (no regex):
AddParty(0)
replace:
( AddParty(0) || AddParty(4) )
... and I think that should be good,
5) In Hades Workshop, "Batch -> Import Field Scripts"; there shouldn't be any error but tell me if there is,
6) "File -> Save Steam Mod" will generate a couple of files but you are interested only in the p0data7.bin (that's the archive containing all the scripts): replace the game's file with it, having a backup for safety.
I *think* this should counter all the problems you can have with this kind of party modifications. Indeed, there are parts of the game (Oeilvert, Ispen Castle...) in which you are supposed to have 4 party members for the cutscenes, so you can't just remove everyone but Freya. The battles that would normally be solo Zidane (Baku, Black Waltz 1 and Amarant... I don't see any other) will be with both Zidane and Freya (again, that's not a good idea to remove Zidane from the team carelessly). And I think that the battle against Plant Brain should be without Blank then... but Plant Brain's AI script seem to be able to handle that case without problem (it will simply be a battle with 1 phase instead of 2).
Of course, you can replace Freya by any other character. In the 2nd and 3rd search/replace,
use "0" instead of "4" for Zidane,
use "1" instead of "4" for Vivi,
use "2" instead of "4" for Dagger,
use "3" instead of "4" for Steiner,
use "5" instead of "4" for Quina,
use "6" instead of "4" for Eiko,
use "7" instead of "4" for Amarant,
use "8" instead of "4" for Beatrix, also use "511" instead of "255" in the "Setting_PartyReserve" line.
I am not sure of how it would work for Quina/Eiko/Amarant because of the shared slot character though...
For replacing the character on the field, that's more complicated because Freya doesn't have all the animations that Zidane has (I don't think that Freya can climb ladders for instance)... It would go pretty much the same way as above but searching for lines:
"SetModel( 98, 93 )", replace by "SetModel( 192, 94 )", but also
"RunAnimation( [Some Animation ID] )", replace with a corresponding animation ID for Freya,
the same for "SetStandAnimation", "SetWalkAnimation", "SetRunAnimation", "SetLeftAnimation", "SetRightAnimation", "SetInactiveAnimation", "SetJumpAnimation", "RunAnimationEx".
The list of animation IDs can be found
there (Freya's animation file names start with "ANH_MAIN_F0_FRJ_" and Zidane's start mainly with "ANH_MAIN_F0_ZDN_" (sometimes it's another number after the "F")).
As you can see, your questions are everything but simple :p
It's a hard problem to determine whether a problem is hard or not.