This answers your questions about the legality.
http://www.dnr.state.wi.us/org/water/fhp/fish/faq/collect.htm
If you obtain small, 1 to 2.5 inch, panfish also known as bluegill, bream, warmouth, fliers,rock bass, perch, green subfish, et al you can keep several in a 10 gal tank. At this size it will be like having a per convicts in your tank and doable. Yes they will grow larger but the size of the tank and the feeding schedule does effect their rate of growth. They don't need to be fed daily. They will eat earthworms, mealworms and feeder guppies if you are concerned about live food. They will eat freeze-dried Krill, tublifex worms, mealworms and can be trained to take flake food eventually. Over filter the 10g tank due to waste issues, keep feeding to a minimum and you will be successful. I have kept successfully them at this size in a 10 gal bucket that was outside for several months at one time barely supplementally feeding them. They still eventually will outgrow your tank but at that time you may have been able to upgrade to a 20L/H or even bigger. A 20H can use alot of the same accesories that the 10g utilized and can be obtained for less than $30 in most places I would think.
For the record I have a 75g tank with two small warmouths less than 2 inches, two large warmouths at 6 inches, a pumpkinseed that is 4-5 inches in size, a 3 inch flier and one 4 inch bluegill with a red crawfish. I also have an Emperor 400 biowheel, another external filter and an internal sponge filter running with several airstones with native Camboda floating. They get fed three times a week (mainly krill, earthworms and mealworms all freeze-dried), have characters like Oscars and nice color.
Regardless, you try two small ones and see how it works for you. Good luck!