If all your good bacteria was in the gravel, it probably would kill the cycle but if you have a filter, then a lot of your good bacteria will be in the filter cartridge instead. If you do a 100% change, just don't replace your filter cartridge for a few weeks.
Depends on how much time you have rightt now. If you have a free morning, you can just do close to 100% and that'll get the salt almost completely out. Or you can do a 100% but I don't know what that will do to any good bacteria you have in there.
Thanks! He's secretly my favourite guppy (don't tell the others xD). :D I think I'm going to put a betta in the 5 Gallon. OFL said I should do a few back to back 50% water changes to get the salt out. Should I do them all in a row, or over the period of a few days?