welcome to the site. I'll try to answer the questions.
planted/community tank. I'm getting an Emperor 400 Filter and 300w heater. I know that the filter will be okay, but is that heater overkill or underkill for a 48" tank? I was thinking maybe two 150w, one on each side or even two 200w. What do ya think?
300W total for the heater should be fine. I'd stick with one heater to make the wiring less complicated.
For lighting I wanted to get 2 18"-24" Double Bright LED lights from Marineland because they look just great. Now with these lights, can I grow plants? Does the moon cycle lighting it has actually do anything for the tank?
Give it a try. We don't have too much data on LEDs
What kind of substrate should I use? I kind of like gravel, but I don't know how well plants can root in gravel. Also a few fish I were looking at like to burrow. Will gravel make burrowing a little difficult?
If you plan to have plants, burrowing fish is a bad idea. Your plants are limited to java ferns, anubias, and moss (plants that you attach to rocks and driftwood. Choose the substrate that you can afford. They can get expensive for a 55 gallon tank.
As for fish, the only thing I'm adamant on is that I want two schools of 10 fish each. I was thinking a school of Cardinal Tetra's and a school of Cherry Barbs. I also want at least 5 shrimp and two cleaner fish (catfish or plecos, which is better for a 55 gallon with a bunch of fish?).
Keep away from the regular pleco. They grow too large and will eat plants. Try bristle nose pleco and ottos.
Final question is about rocks. I don't really think it'd be a sweet idea to pay for rocks. Are there certain types of rocks I shouldn't put in my tank? There are a few real nice looking decorative rocks in the yards around my neighborhood I'm probably going to end up taking. If I cycle them in separate water, are they okay to drop in?
Keep away from limestones. Wash the rocks thoroughly to get any insecticides/pesticides off the rocks. Soak the rocks in a bucket of water for 1-2 weeks. Then test the PH on the water to see if it's too high. Stay away from those rocks if it's too high.
I'd recommend reading the plant guide for further details or do more research on the forums.