Just wanted to say not to get 'pairs of livebearers'. They should eb kept as trios - 2 females for every male. Also, not all livebearers are peaceful or compatible. Guppies, for example, are often bullied while mollies are often bullies.
In a 55 gallon, I like the boesmani rainbows. Also take a look at the turqoise rainbow (melanotaenia lacustris) which is a lovely bright blue. You could keep both species. They get to 4" and should each be in a group of around 5 minnimum as they are schooling fish.
That would still leave you with a little space which I would fill with a few interesting loaches (try botia striata - zebra loach). Alternatively, a big school of some kind of corydoras catfish (I like c. aenus and c. leukomelas - bronze/albino and blackfin respectively. The peppered cories are also quite hardy cories if that's what you're looking for.). Cories should be in a group of 4 or more. Idealy, choose between the bottom dwellers - either go for the cories or the zebra loaches (with other laoches its ok to mix - I just eprsonaly have had some bad experiences with this particular combination). Another lovely loach is the yoyo which is less aggressive than the zebra towards other bottom dwellers and is actualy quite social with its own kind IME.
Alternatively, you could go for just one of the rainbow species or try the neon/dwarf rainbow (melanotaenia praecox) to leave yourself some extra room for other fish.
Regardless, you may also want to put in some kind of smaller schooling fish or a larger 'centerpiece fish'.
Good small schoolers are harlequin rasboras, rummynose/firehead tetras, black phantom tetras or any of the danios (except giant which tend to be nippy and grow larger). All these should be in a group of 5-6. All of these get from between 1.5" to 2".
Centerpiece fish to look at are rams, I preffer bolivians, as a pair. There's also many countless gourami species that would look fantastic. If you take care to not get anything nippy, a trio of pearl gouramies (trichogaster leeri - to 5") would look amazing (especialy if you plant the tank). Other gouramies you can look at are: moonlights (trichogaster microlepis - 7" - as a trio or singly), three-spots (trichogaster trichopterus - 6" - either keep several females or a single male - don't keep more than one male or other gouramies with him - there's a number of color morphs to choose from such as the opaline, blue, gold or platinum), banded (colisa fasciata - 4"), thick-lipped (colisa labiosa 3.5") and dwarf (colisa lalia - 2" - often confused with the honey gourami which is colisa chuna/sota and stays smaller - the dwarf comes in a multitude of color morphs of which the males are the bright ones. They are unfortunately prone to disease.)
Those are all very colorful, attractive fish.
There are some other fish to take into consideration also such as keyhole cichlids (and other dwarf neotropicals besides rams), small to medium-sized plecostomus catfish, certain other catfish such as upside-down cats, various shark-like species (siamese algae eaters, false siamese algae eaters, oblongus and flying foxes are all quite peaceful and get to about 4-6" some of which should be kept singly but there's also the more aggressive red tailed black shark and rainbow sharks which need to be kept singly, in a large tank and may not be suitable tankmates for certain fish).
The fact is, there's hundreds, if not more, species of fish to choose from... Research whatever you are interested in slowly because I guarantee you'll regret it otherwise. Seriously, take your time, you'llw ant another tank before long...
Oh and here's what I would do in a heavily planted 55 if I wanted to meet your criteria:
7 praecox rainbows
3 opaline gouramies (trio)
3 yoyo loaches
1 flying fox
5 (yellow mickey-mouse) platies (2 males, 3 females)
BTW, there's useful GUIDELINE (it's not a rule) that says that for every gallon of water you can have one inch of fish. The 'inch' is meant to reffer to the adult size of the fish - so a 2" opaline gourami counts for 6" because that's the size it will reach. The guide is not actualy emant to be used for anything over about 4" and only applies to slim-bodied fish like neon tetras but it is still a good guide. The gallons are in US gallons (as opposed to UK gallons).