Ludwigia sp. Red also works in a small tank, as would AR Mini.Most Rotala species are well suited for smaller tanks, due to small leave sizes. Some of my favorites are R. Colorata, R. Magenta (though I haven't had the best luck myself with it), R. Wallichii, and R. rotundifolia. Those will all contrast well against each other as well.
I find Alternanthera reineckii does well up front, it's not a super slow grower but it's not fast. If you just trim the very top, it seems to bush out which can make a good midground plant. I have not tried AR Mini which is a popular plant right now.
Basically anything you like should work and I find red plants are easier in shorter tanks, due to the fact the light closer. The only consideration for a 5.5 would be leaf size and how the plant grows. Many Ludwigia species can be hard because the either have big leaves, they like to grow sideways under high light, the like to grow into big bushes that may take up all your tank, or all of the above. However, L. arcuata may be a good choice as well, often sold under "Narrow Leaf Ludwiga".