Pakash - not sure about the combination, but most people on here suggest keeping Dems in larger groups. I think somewhere around 15 is ideal to spread their aggression. Also, unless you know the sex of the fish you are getting you'll probably need to start with more so you can work your way down to the desired M/F ratio.
Regarding your filtration, will the HOB and sump be dedicated to different filtration types? Seems like you could get away with the sump alone if you set it up right.