I would add a few more at once. I had three a few years back and then added another three many months later. Unfortunately most of them ended up jumping over time. I would make sure you have a tight fitting canopy or at least some kind of mesh top. The only thing about a mesh top is that they are so small and can usually get through.
I never had problems with them fighting when adding new ones. Never heard of problems either. I think they are quite peaceful. Too bad they are such jumpers!
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