Quite a menagerie you've got going there!
Your bettas are lucky if you're buying them each a 6 gallon JBJ cube: those tanks are expensive, but you get what you pay for. Considering how many people just throw bettas in fruit jars or water glasses or whatever, anything
bigger is a step up.
There are a lot of threads in here about substrate, plants, filtration/water movement; a lot of people have strong opinions about this being better than that or what have you, so expect more replies than just mine; but I'd say for plants you should start with fast-growing stems (moneywort, wisteria, maybe a rotala) since they contribute to healthier water, look great, and give your fish something to hide out in. For shorter plants steer towards things that don't need intense light - java moss for instance.
As for an airstone... it can't hurt. Seriously.
Lastly, rather than risk starting an endless back & forth debate, I'll say that my
personal preference for substrate is ADA Aquasoil (granted, it's the only thing I've used yet though). It's nicely monochrome and has a lot of nutrients to get your plants' root systems started. Many people find it too pricy for their tastes, but like I said before, if you're springin' for two JBJ 6g cubes I think a $20 bag of substrate shouldn't be too much of an issue.
PS - be prepared for some folks' commentaries on your old betta tanks' decor in the pics you posted.