well, if you are going community... I would say in the 55 that the following are cool fish I would want in a community:
1 or 2 Golden/Blue Rams (south American Cichlid)
Rosy Barbs
Head & Tail Lights (sorry, don't know specific names)
Bushy Nosed Plecostomus
Armored Catfish
Neons
Cloud Tetras
Danios
Swords
Now, unless things have changed, it's 2 inches of fish per gallon of water... I think you can fudge that either way, obviously, for something large like an oscar you want less fish, more water they are massive creatures, but for something like a neon, you don't have a lot of mass and can probably put a few more in than you think.
Remember to add slowly...
Heres the thing though... with fresh water communities you have a couple of choices... chichlid community, predator community, non predator community, breed specific community, and flat out put what you like in it.
I think for your 55 you should go for color and movement, don't mess with live plants, put in silk plants, fine gravel, and some well washed bog wood. add some small snails or apples as well, if you don't mix with the apple snails, you could get a Hammers Cobalt Blue Lobster which is relatively peaceable for a crawfish, and won't eat your fish. (as long as they are fast enough to get away.)
For something truly dazzling, your 90 gallon, I think, especially since your here on a plant lovers forum, you should do a planted tank. They are harder to keep, but are worth the reward.
You will want to put a product like flourite in as your substrate (i think covering the flourite with a normal gravel is a good idea too, helps to control algae.) Cycle the tank with a fish you want to keep, I would reccomend a cat fish, they are tough little suckers...
Then put in amazon sword and val (there lots of varieties of val that are gorgeous, including some cork screws) put crypticorns and some other midlevel plants infront of the taller swords and vals, (don't forget bog wood) and the cover the bottom with micro-sword (these are easy plants to start with, when you can keep them alive, you can move on to some of the harder stuff that is really awesome! (not that these plants are not awesome.)
for invertebrates, I say malaysian trumpet snails, they will breed in the substrate and provide algae control, also stirring the substrate keeping it aerated. For Fishy Algae Control, about 10 SIAMESE algae eaters. It is important you get siamese, as chinese will kill each other, and don't eat black hair algae. Siamese algeae eaters are essential, tiny, and you won't really notice them unless you look for them. Put in a few cory cats, and then, the apex species of your tank, discuss. They are the crown jewel of many eyes for fresh water aquariums.
Couple reccomendations when doing a planted tank:
use RO water if you can, especially if you can UV sterilize it... sure it's going to get dirty in the tank, that's fine, you want to minimize the transport of diatomic algaes into the tank, since you will be fertilizing it... eventually, your plants will soak up nutrients in the tank like a sponge, and out compete the algae, but you will want every bit of help you can get.
CO2 injection is a great way to make your plants grow, and there are simple ways of doing it. If you are injecting CO2, you will want as little disturbance of the water as possible during the day, and when the lights go off, you will want an air pump to come on (as plants breath the same as animals at night, Oxygen in, Co2 out... during the day, it's Co2 in, Oxygen out. This keeps everyone happy in the tank.
The discuss will appreciate a planted tank and will look beautiful against the backdrop. I would reccomend no more than 4, and keep the tank lightly stocked, a few neons or neon like fish that school would probably look nice as well. I have always felt if your going to put neons in a big tank, you should put in enough to get schooling behavior.
Is that enough of a reccomendation?