I wouldnt suggest keeping them with any Damsel (except chromis)
black and gold chromis http://www.fishbase.org/images/thumbnails/jpg/tn_Nenig_u4.jpg one of the meanest fish under 5'' or Dotty,both are agressive lil species and will give the fish a hard time.
i would hardly consider orchid, sankeyi, aldabraensis, or indigo dottys agressive, especially if added later or last to a tank
On Stocking i would say Maximum 2 per 20 gallons with one other fish etc....
I give 5 gallon per fish as a rule depending on size,
(well then it's hardly a rule then is it?
) you could have 3 firefish in a species tank 20 gall.
As far as i have read they prefer being on there own anyway :S i may be wrong,i would research that one.