I don't think it's the ocarina codes.
I think that the motion file has a very anal file size limit as well.
Check that file out too.
Also, the tracklist modifier can cause freezing at the SSS and title screen to main menu at times.
Ok just ran a few tests... and I have narrowed down the cause a little... maybe.
I downloaded brawl minus, installed it on the SD card and ran it with the riiv codes that came with it. Everything loaded and samus worked just fine (255 kb .pac file with brawl- btw). Just for kicks I took my custom codes .gct file and made it load with brawl minus... still no problems! Everything loaded just fine, which means you were right about it not being the codes. And samus didn't freeze on the selection screen.
Next, I took all my custom samus stuff out from my custom mod fileset so she was just vanilla (even replaced her fitmotionetc in common_en.pac file with the plain one just to make sure it wasn't that). So she was back to normal, totally. Then I took the fitsamus.pac file from brawl minus and dropped it in her fighter folder... and the game froze at the selection screen just as usual when I loaded my custom mod.
So we know it's not the codes. We know it's not the fitmotionetc... but for some reason when I load try to load a fitsamus.pac file greater than 253 kb in my custom modset with modlauncher in riivolution, it freezes. The only thing I can think of is that it has something to do with the common5_en.pac file that the Flaming Blue CSS uses. I had this error before ever doing custom stages with tracklist modifier, so I know it isn't that either.
I guess my question is now... has anyone here gotten riivolution to load a custom fitsamus.pac file greater than 253 kb AND is also using the flaming blue CSS with all the extra characters? If so, could you please contact me so maybe we can exchange some files and see if I can get this working? I'm at my wits end here, it seems like I've tried everything.