Also - a word of friendly advice - you probably should investigate the plants on this or another site before buying them to make sure they are compatable with your growing set-up. Some have easy demands - other have difficult ones, etc, etc.
I know its easy to make an impulse purchase at your LFS - but in the long run, you may be better off researching and planning what you want and what will do well first
Rotala wallichi is a toughie, I grew some in a 6 gallon with DIY co2. Whenever co2 output was good, it grew well, but when the output fell, the rotala stopped growing and died back.
I'd go with Myriophyllum as well. It's more of a tangled big mess, while R. wallichii is skinnier and not as bushy. Also, R. wallichii doesn't have the red stem.