One word of advice, you might want to start one thread and stick with it. That will likely give you better responses than scattering bits of info over various thread and forums (eg
here and
here), so nobody gets a clear picture of your situation.
To summarize, you have a 55G - beautiful setup by the way :thumb: - and have decided on keeping peacocks. You want a colorful and active tank, and are wondering how to best stock it on a budget.
To expand a bit on what DJRansome wrote, before peacocks color up, there is no sure way of telling what's a male and what's a female. Also, all female peacocks look so similar that there is no sure way to tell them apart once they swim in the same tank. Finally, producing hybrids should be avoided at all costs, since they can spoil the blood lines many people take great pains to keep pure.
What all this means in practice is that if you buy baby peacocks you should only buy one group that you know all to be the same species to house in one tank. They should never be combined with other peacocks. A group like that would make a nice breeding setup in your 55G. You would get to observe a lot of the peacocks' natural behavior, but likely only your dominant male would color up.
In a pinch, you might be able to combine them with other peaceful fish. Most mbuna would be too boisterous, but maybe something like L. caeruleus would work. They would definitely ad a great color contrast to the blue of many peacocks. Hopefully somebody more experienced with peacocks can comment on this option.
The only option to get a more colorful tank with all peacocks is an all-male-setup. For that you have to buy colored up males, which would obviously be more expensive. Another disadvantage is that you won't get to see as much of the natural behavior of the peacocks, because the females are missing.
Last but not least, if you haven't discovered it already, I think you will find the
Peacock Corner of our library worth exploring.
Best of luck!