Cabinet plan looks pretty good - you should be able to have it pretty tall, since the 40B is such a low tank. Several sites online sell many different types of natives - some, like the darters, minnows, and various native sunfish can be quite colorful. One of the major issues with natives is their requirement in most cases for live food, however some can be converted to frozen or even flake food. If you collect your own, always be sure and do so ethically, follow local laws and regulations regarding collection of native species, and never ever return natives or nonnatives to the wild, after they have been in captivity. It is also not advisable to mix natives with tropicals.