as far as cycling goes, if your 10 has stabilized with the canister filter, take it and put it onto the 20g, it's already cycled. but you might want to squeeze a bit of the filter sponge's gunk into the 10g's filter which you should put back onto the 10g. When you move to the 20g, take all the fish with the filter. It is already loaded with the bacteria for the bioload the fish put on the filtering system. Its more or less cycled as long as you keep the filter material wet.
plants i would recommend hydrophillia polysperma, java fern, java moss, crypts, and anubias, and vals. These all grow great with low to med. light.
BTW what is the lighting setup on the 2 tanks? this is quite important for plants to survive in. You don't want too much or too little when dealing with plants.
for a 20H go with a 2 tube t8 or t5 normal output, this'll give ~med light. or you can hack n mod 2 cfls in there. Either putting them horizontally within the cover, or the better option is to have them hang vertically above the tank. I've found that vertically is MUCH better than horizontal. I currently use desk lamps to light 2 of my tanks and they are all doing wonderfully. The modded tank hood with 2 horizontal bulbs is only doing ok.