I will answer most of your questions but also refer to the post Article Critique (has an article on beginner brackish aquariums. You will be able to have several tankmates in a 55. I actually just re-set up my 55 Brackish tank (filters are all cycled so get to stock Tues!) but the tank already has 4 Anablep Anablep (2 different species) and this is by far the coolest and most odd brackish fish.... wait the oddest fish ever since its pupil in split in two so they can see above and below water at the same time. They are extremely friendly and always attack my hand at feeding time. I also have a little Mudskipper in there. Along with several baby mollies. Pretty much your options are open. I will list what I am going to house and you can get some ideas (by the way filtration is two Penguin 330's). There will be the Anableps, Skippy, 2-3 Archers, 4-6 Mono Sebaes, and some bottom gobies. The Monos will get moved to my 209 saltwater tank when older so it will be stocked for awhile but not forever. I would say stay away from Scats, Monos, Datinoides, any anything else that gets over 6 inches or requires saltwater as adults. For live plants in mine I have 3 Java Fern Lace plants. You can have Anubias, some Vals, and Java Fern are the best options. Use Instant Ocean (made by Aquarium Systems) or some saltwater salt of the like. I don't think it really matters on adding plants before or after cycle.