Your fixture should be great for growing live plants! Very good choice on the light :)
Good tall plants that I can think of are Amazon Swords (bit of a slow grower, but once it gets going, it gets HUGE. Very tall with lots of leaves reaching outwards.) Another great one is Crispus Aponogeton (most other varieties of aponogeton work great, as well.) I have this plant in my tank and after a few weeks I had to trim it. I measured...the leaves were 3 feet long! They also grow quite quickly.
Jungle val would also do the trick, and can multiply quite quickly. There is also crypt. Balansae and Crypt. Spirlis, both are very pretty grass-like plants that can get to a decent height, but may not reach the top.
For the betta tank...what light is on that one? A few good plants I can think of are Dwarf Sag, Crypt. Lucens, and Pygmy chain sword.
Floating plants are also a great option for both tanks, many fish really appreciate the sense of 'cover' it gives, and love to browse through the roots. The betta will especially like this :) My favorite floating plants are frogbit and watersprite.
What substrate do you have in the tanks? I recommend adding root tabs in sand (and fine gravel) substrate, as many plants are heavy root feeders (especially the swords and crypts!) and won't do so well without tabs.
Oh! Before I forget, one last thing you'll want to invest in as a bottle of Seachem Flourish Comprehensive plant supplement. It's a great fertilizer that only needs dosed once a week, and will last you months.
As for CO2, I don't run this any any of my tanks and they're quite healthy with great growth :)
Experiment with different plants, it's fun to re-arrange them and watch them grow. And if they don't, that's okay! There are always a few finicky plants out there. :)
Good luck! If you have anymore questions feel free to ask :)