My discus have had harleqluins, cardinals, rummynose, sterbai, king tiger plecs & clown loaches with them. They have never worried about the clown loaches although the clowns did tend to zip around the tank at times. I found the Bristlenose were fine when young but when they matured they could not cope with the high tank temps. The harlequins, cardinals and rummynose also grew fast and had a shortened lifespan, once again to the higher temps.
They are most definitely better kept in larger groups as it spreads the aggression. What you do not want is 4 or so males and 1 female as that is serious fighting material.