Generally speaking, the scaling is working off item tags,
Ex:
"itemTags" : ["weapon","melee","whip"],

and some specializations are looking at projectile types ('bullet').

I'd look through the Git to understand things more explicitly if you're looking to ensure compatibility, but you generally can't go wrong if you make sure your weapons follow the same format at the ones included in the mod.

RPG Growth currently works with *almost* every weapon in my combined Shellguard/FU/Elithian modset, and the ones it doesn't work with are just issues with tagging differences between mods.