Freshwater aquarium fish questions.?

I live in Baton Rouge and, as you might imagine, there are numerous bayous and creeks that run throughout the city. Many are left 'natural,' that is, with mud banks as opposed to those ugly concrete ones. In the ones by my house I can see many beautiful guppies and other fish I can't identify (I am very much a beginner w/ aquariums) that are colorful and of the right size. I can only guess they are the descendants of released pets.

Would it be advisable to catch some to add to my tank? What precautions should I take?

There are also crawfish, though none of the blue or white ones I see in shops/online - just plain, edible crawfish. I've even seen tiny frogs, but am unsure if they are not just juvenile bullfrogs or leopard frogs - the most common varieties here. Newts, as well.

On a different note, I've always liked frogs and toads and have toyed with the idea of raising either a bullfrog or a common toad. The toad would be easy, once I learned the ABC's (I'm guessing) but a bullfrog could be a problem. Anyone know how often they'd need a water change and how fast they grow in captivity? I've seen some truly massive bullfrogs out in the Atchafalaya Basin. As in ones that looked to weigh 8 pounds and approach the size of rabbits.

Alligator snapping turtles would also make an interesting pet, though a friend when I was a kid tried it and the water became a muddy, smelly mess quick - though he used no equipment besides a tank. Anyone know anything on that?

THANKS!