Cories & kuhlis do fine on smooth rounded gravel, although sand is especially appreciated by the latter.
I think the visual appeal of one large school is much nicer... Only 7 rummies? Not enough in a 90, they'll be lost visually. I'd say the OP has it right, 15-20 at least for a school - a single school of 30 rummy, cardinal, neon tetras or harlequin rasbora would be great... But with angels you may need deeper bodied fish, so I'd lean towards the harleys. And I'd say at least a dozen albino aeneus, trilineatus or sterbai cories.
I think the visual appeal of one large school is much nicer... Only 7 rummies? Not enough in a 90, they'll be lost visually. I'd say the OP has it right, 15-20 at least for a school - a single school of 30 rummy, cardinal, neon tetras or harlequin rasbora would be great... But with angels you may need deeper bodied fish, so I'd lean towards the harleys. And I'd say at least a dozen albino aeneus, trilineatus or sterbai cories.