Correct. Only separate the stem plants and replant the individual stems if you'd prefer. Personally, I like to bunch them together too (3-5 stems per bunch) and space the bunches a couple of inches apart. You normally don't see problems unless you plant too many individual stems together. (3-5 is fine.)
Not sure what the grass looking plant is but it looks like Mondo Grass which isn't a true aquatic plant and will eventually die. If you can positively ID it as Mondo Grass, you might want to remove it and get an aquatic species to replace it.