Personally, I just see it as SeiGou V2
I mean, Momo and Sei look a lot a like, act similar(Momo is more straight forward), and have incredibly similar taste. So in all honesty it’s really just two versions of each other, just that one involves the older brother and the other is the younger brother.
I ship Gou with Makoto as an otp, and personally I think SeiGou is okay, so I side ship it, and MomoGou will most definitely fall into the same category as SeiGou for me, they’re all super cute.
Or we can all just ship the four of them together, give Gou a harem of sexy hot muscled backstroke swimmers.