Hello
Firstly I would never allow any parrot specie in with diamond doves. I have an aviary of them and they are so small and delicate that they will easily be killed by these. In fact how I actually ended up with my first ones is that someone had these birds together in an aviary and the cockatiels kept killing them. They tend to not fight back unless its another dove so they just allow themselves to be picked on unfortunately.
A pair would be great. however in the wild they naturally live in flocks kinda like feral pigeons do. only with a different diet

. Once I had mine in a bigger group they seemed to be much more happier and two pairs will even share a nest. Just to show how docile they are.
Parrots like cockatiels are very intolerant of straight billed birds and should only be kept with birds of the hooked billed varieties like themselves and of a similar size. However, if this a cage environment and not an aviary them one of the same species but opposite genders is way he/she should be housed with.
Hope this helped