A community setting is ideal to help form the pair. But keep in mind that festae usually don't pair for long and may need to be divided or swapped out with opposite sex. Really any dithers work as long as the festae are your main focus in the tank. The 150 will only be large enough to house an adult festae pair and no others. The 600 is good if you wanna throw them all in there together and later move the festae. But if your looking to keep the festae as a pair then try keeping them in the larger tank with community. You may also wanna remove any fry immediately to help keep the peace from any pair that spawns in the commun ity.