How long would they be in the 15 before transferring over to the 90?
I'm trying to pair up 2 grammas now. I did a lot of looking into how to do it. Most accounts said one small and one medium. So that is what I did. They are in my 16 gallon qt tank now for 5 days. One is out a good bit, one hides a good bit. They do seem to get along and even share a cave. Fingers crossed.
I'd say if the long term plan is to move them into a 90 then go for it. Just be very observant and if any aggression starts up be ready to move one.
I found conflicting information on if they are protogynous hermaphrodites or determinate. Initially what I was seeing is that they are protogynous, but later and on mofib it seemed to indicate that was not true and that they were determinate... so I don't know that I would rely on the fact that they all start out as females.
Know that is not an answer, but that is what I found. If someone DOES know the answer, please share!