Riccia IMO is going to be a PITA in the long run with it's frequent trimming schedule with such high light an' CO2 injection. Micranthenum micranthemoides - good plant for a BG responds quite well to trims. would like you to give HM a thought, rotala sounds good too but it's growth in conditions you are ready to provide will be like it's on steroids..hehe. If you are going for a mix carpet - put glosso infront with HC a bit back..will add a lot of perspective depth, parvula in small quantity in spaces. Ammania 'bonsai' is a good choice for progression towards your stems. Styruogyne 'repens' is a good choice as well. But to all these it all depends upon the scape that you are trying to come up. A sketch perhaps can help people suggest plant choice better..hope it helps..