3, maybe 5 pygos max in a 90....Most likely you'll be upgrading, so start off with 10 or 12 dimers.....90% of all pygo setups do start off in 60g or less....And end up in 120's or longer.....
As for serras, I never had one....But sounds like a 90g would be nice for a single serra(for life)....I was thinking of adopting a black, but once I heard how slow these guys grow, I pretty much tunrned away from the opportunity and went full hardcore on Pygo....