it depends on the male to female ratio. the males are territorial and will fight to the death in to small a space. i tried four in 10g. they all turned out to be male. only one survived. i currntly have 6 in my 33g community tank. 3male 3 female. they are doing fine in this size tank. not sure on CPD as tank mates i think they prefer harder water to sparklers. i keep mine with cardinal teras and german blue rams.
I agree. First issue is tank size, and then male/female. I also have a group of six in a 33g, there are two males and two females that I know of because they have spawned at opposite ends of the tank. The remaining two I'm not sure of, but in this tank which is thick with floating plants, they are fine.
The Harlequin Rasbora would be fine with the gourami. I have a group of Trigonostigma hengeli (similar to the slightly larger Harlequin) in with mine, and a group of Brevibora dorsiocellata (the "Emerald Eye" Rasbora).
Celestial Pearl Danio would be better on their own, in a group of 20. Dominant males can be feisty, so the larger group can spread this out.
"Peaceful" is a subjective term, but yes, compared to many other fish, gourami are generally peaceful. But they also are quite close to cichlids in behaviours, in that males are territorial, sometimes very strongly so, depending upon species. And another factor is where in the water column other fish live. Gourami tend to remain in the top half, sometimes the top third, of the water column. If you have male/female, they will inevitably spawn. The male can be very protective of eggs (which some species including the sparkling pygmy gourami place in a bubble nest at the surface in floating plants) and I have seen other surface or surfacing fish get nipped quite voraciously even by these little gourami.
Sometimes a group is best, sometimes a pair, sometimes a "harem" (one male and two or three females); it depends upon species. In the case of the sparkling pygmy gourami, if the tank has sufficient space, I would get a group of five or a few more. In a smaller tank, a trio with one male and two females works well; distinguishing male/female is not easy especially in juvenile fish. I have not had problems with just three or four fish, but only in a 20g or larger tank,