Kou did some testing with the code limits
here. I thought it was 255 lines (idk what size that gct would be), but I've also heard that booting with cheats from USB can handle less lines than booting from an SD card.
For your second question, you could use the D-Pad Down toggles invincibility code and practice stuff against an [invincible] cpu or human. The "invincibility" is just no knockback. Another good way of practicing this with the given codes is to start a game using D-Pad Up to load the stage select screen, while making another player be Master Hand (I give details of how to do this in the original post, and its in the gif). Master Hand suffers no knockback but gives you hitlag as if you were hitting a regular player.
Thanks for notifying me of this. I'll try and look into it.