Hehe, a seperate tank for plants. That's an idea. I made the mistake of putting some plants (albiet rather inexpensive ones) in my RESs tank just to make it look less... bland i guess is the word. Within minutes they were gone. Granted she's pushing 12 years old, and nearly 8-10" long. We feed her small feeder fish that have been QTed for at least 2 weeks before we give them to her. She also gets a good amount of veggies and pellets.
I think that if your turtles are fans of anachris or elodea, that having a 10gal tank setup with a decent light, you could clip it back every few days (or if theres enough ever day) and toss it in. Like others have mentioned, I think that if you are only feeding from the 10gal that you'll need a larger tank. You could also throw a pair of breeding guppies into the tank and have some supplimental "meatier" foods, although, if you have the space a larger tank would server the purpose more. a 20 long is only 10" longer than the 10, and is my favorite tank for such things, though my brother does his guppy breeding in a 5 gal for his oscars.
I'm curious, and do not mean to be critical at all, but what are your future plans for the turtles? They do get large, and its a case of "the more room the better". We aquired a 75gal tank for free, and upgraded our turtle from her 55 to the 75. She's much happier. Then again, like I said, she's nearly 12 years old. Sounds like you got a good handle on this.