you can add plants now and they should be fine if you add plant food. i don't know how much they'll help with cycling, but they keep
nitrate levels down. as far as what types to get, you'll probably want lowlight plants depending on the wattage you have now. i think less than two watts per gallon, and definately less than one watt per gallon is considered low light. java moss and java ferns would be good and easy to take care of. there are others types of easy low care plants also, but i'm not that well versed in this yet-- plantgeek.com is a good site and will answer all of your questions!