Furcatus are fairly active so I'd probably only do maybe 4-6 of them. They are very similar to danios in behaviour to give you an idea of aggression and activity.
Gertrudae are slightly more subdued and so you could probably house 6-8 comfortably in a 10 gallon tank. They do like some areas of heavy cover to retreat to, but once they have settled in they are generally fairly bold and mine all used to come up and nibble on my fingers when I stuck my hand in.
This is one of the set-ups I had mine in (tank is a 15 gallon) and they thrived.
Here they are along with my rainbows eating some grindals.
Like most smaller species, they can be a little fragile at first. However, if your tank is mature and cycled they should have no problems adjusting. Mine were hardy as heck and thrived in my large community set-up.