I was fine with including in the list also those where Squaresoft was just the publisher and not the developer
Extracting from other non Squaresoft ROMs will likely not provide easily the 3 types of data required to make the sample sound good in FF6. So one avenue could be just ripping BRR sample by whatever method (SPC files, SNESSOR95, etc.) and then write manually the ADSR (like a default FFE0), pitch and loop data for each sample. [...] Ideally all game packages should have a list of FF6 compatible pitch data but for now it is the pitch data from the game samples were extracted.Sorry but you're contradicting yourself here, man. Games developed by other companies and published by Squaresoft, such as Alcahest or Breath of Fire (U) are not using any of Akao's sound engine and are not going to be directly insertible in FF6's engine.
Same goes with games developed by Square but not using any of Akao's engines (Secret of Evermore, Seiken Densetsu III, Bahamut Lagoon and Super Mario RPG); also games using older versions of Akao's engine such as FF4, Romancing Saga, FF5, Secret of Mana and FF Mysqic Quest will have their pitch regulated by a single byte instead of two bytes (I think); FF4 doesn't even use ADSR at all and use volume lockup tables instead so ADSR won't be compatible either.
None of this prevents you to re-use/import any sample from any SNES game, though, however, as the loop data is hardware standard so all SPC engines are using the same format, just like the BRR itself. As for ADSR it's a non-issue since you can make your own up very easily, so pitch is the only really problematic instrument parameter when copying from another game - it's quite easy to fix an instrument with trial and error but this requires that other instruments are already correctly pitched.
Also considering how simple it is to convert a non-BRR sample to BRR thank to my amazing utility, I don't see why you would limit yourself to BRR smaples :p