The "Enjoy!" would suggest he is making it free for use with credits given. Seeing as how he admits it's a SOTN edit, it's not like there's a whole lot he can do. Just give him credit and give Konami credit and all will be good. If he even notices you and throws a hissy, you could just threaten to tell Konami that he's claiming their work as his own. Custom sprites are a tricky, grey area. But looking at that sprite sheet, he doesnt' strike me as that kind of person. After hanging around the Spriters Resource for a while, you learn to read people's personalities just from their sprite sheets. All they want is credit for going through the effort of making minor edits for your sake.
Still looking for Succubus put together, but for now here's the succubus from Demon Hunter Legend:
SOTN:
Can't find anything even in Japanese. Probably not looking with the right terms.