if you are going to do a 30% waterchange, you might as well do a 50% waterchange.
That'll only take you 10 minutes extra.
That way you'll be able to cope with the bioload, imo.
And your Oscar will love to chase the SD's around.
Plus they actually do a very good job cleaning up after the oscar.
I do not specificly feed the SD's, but they'll eat any scraps from the oscars, pleco's and even the fish i feed to my gar.
I'd say get one male and 3 females, and you'll have yourself a good combo!
4 is a good number for SD's.