8 would be fine. You have the space for them bioload wise & your filters are up to the task. 8 will also have them concentrating on each other more rather than your other fish becasue like Ryan says they can get a bit nippy.
Thank you all, I believe I will get some next time I visit my lfs
Should I get 8 all at once or only 4 at a time so it won't mess up the bacteria in the filters
Merged back to back posts.
I would get all 8 if they are still small (juveniles).
They will have strength in numbers, and will more easily establish their 'pecking' order.
Your current setup should deal with the extra bio-load quite quickly (about a week max probably).
Just keep an eye on your ammonia readings, and if they rise, just perform a water change to reduce the concentrations.
Back to the pH question... I have four Bleeding Hearts in my tank. They are healthy and hardy (surviving 3 major moves, temperatures at around 60 degrees for months, etc). The pH of the water here is just about as high as it goes. From my personal experience, Bleeding Hearts should do fine in hard water. They're a good-looking, tough little fish.