Please don't put Feraligatr as Wally's FIRST battle at Mauville City. A Croconaw or something would be acceptable, but not Feraligatr. That is just overkill.
Also, if you're going to change Wally's starter Pokemon to Totodile, make sure to also change the script on Route 102 to make him catch a Totodile, and to be consistent, you should also make Totodiles available on Route 102. There are a lot of things you have to consider when making a ROM hack to make it make sense.