J. dickfeldi is a dwarf julie, so they don't get as big as say the regani or marlieri. With no other bottom dwellers in the tank I would say you could easily get away with 3 maybe 4 adult pairs with proper rock work, though if they all breed at the same time you'll have some problems on your hands. A 55 can make a decent mini Tanganyikan community. So with the end result in mind, 2 pair of julies would be plenty with another type of rock dweller or some of the larger shellies. A nice school of non-jumbo cyps would be very nice for filling out the upper tank giving you a 100% cichlid tank that you don't see every day.
And not tring to come down on you cause your young, this is just fair warning. Some Tanganyikan's can be pricey, so that can easily adjust your stock for you.
But the julies are fairly common, so you shouldn't have much trouble with them. The 'midnight' strain of the dickfeldi are beutiful if you can find them.