I have kept a flame,a herald's and a keyhole in a 55g before. I added them simultaneously so as to avoid any initial territorial establishments among them.Have since transferred them to a 65g housing a Juvenile koran that is only slightly bigger than them. The flame tends to be the aggressor among the 3 dwarves,however,squabbles are rare and the other 2 dwarves can hold their own.If fed adequately, fights could easily be avoided.Good luck.One more thing, dont try the bicolor angel, it is way too docile and is easily bullied.