Well, a lot depends on what type of light you have. Many of those plants need medium to high light to really look good. What type of lights do you have? Because your tanks are smaller, I'd guess that you have the lights that came with the tanks? THat's probably low light, and most of these plants will not grow well for you.
Here's some low light plants that look great and are super easy to grow:
-Anubias (this is a rhizome plant, only bury roots not tuber). Many types.
-Java Fern (also rhizome, looks great tied on driftwood). May I suggest the 'windelov' variety?
-Cryptocorenes (rooted in the substrate)
-Java moss (a moss, grows like crazy in anything)
-Marimo moss balls (great anti-algae, since it is an algae, just one you want!)
I would not buy a large collection of plants like that just because you could have massive failure and lose all that money. I'd suggest you buy a couple different types of plants, research them, and see how it goes. Best to start small, figure out what your capabilities are, and go from there.
HTH. I may be a noob about bettas, but not about plants!
I'm actually not so sure on the whole 75% stem plants thing.. I'd encourage you to get 3-4 smaller crypts (3 crypt wendtii 1.5 inches apart in a corner, and crypt willissi or parva across the tank from them toward the front) for the 2.5 and crypts and small swordplants for the other (Crypt wendtiis, blassii, spiralis, and a echinodorus parviflorus or three) with some nice stem plants as filler.
Hygrophila sp, ludwigea repens, rotala indica, proserinaca palustrus, and a few others would work well. Essentially, if its red or pink, don't expect it to grow.
I thought water lettuce when in the tank it doesn't get enough light so it is called "dwarf" water lettuce? When given full sun they should grow into larger plants, but that requires a lot of light. This one lady, Diana, was talking about it when I went to a SFBAAPS meeting to her house, she just took them from the delta XD