For the record, it's clear they designed 18's super before 17 became a main character. So I wouldn't be surprised to see a lot of fan service for him in her super attack. But that just makes the attack cooler I guess. And they can give him another team attack with 18 as his super to make it even.
Yeah, it's all dependent on how much 17's a part of 18's moveset. Initially I was thinking about him being implemented like Sorbet is in Frieza's moveset - a pseudo-assist character - but if it is just a super move (which the Budokai series also did) he'd probably be DLC.
As per your suggestion it could even end up being cool if they're a part of each other's super move, especially if they maintain their palette if they're part of your team. It could be a good way of showing their special synergy.