i agree that your 6g is too small for 6 goldfish.. nevertheless just one goldfish. Even a feeder goldfish would grow too big for a 6g.
Plus goldfish tend to eat plants, so if you would really like to grow some healthy plants, i'd skip the goldfish and perhaps add some small schooling fish (like White clouds, harlequin raspboras, and cherry barbs <not schooling, but does well in planted tanks&gt
.
how much lighting do you have in the 6g?
Some low light plants and the java moss and java fern would do well for you. I'd also add some crypts and anubias to the list.
Any other plants would depend on the amount of flourescent lighting you are using/going to use.