I am sorry, but no turtle is happy in a ten gallon tank, unless it is a BABY mud or musk or something similar sized. I feel horrible for any turtle that has lived in a ten gallon for six years. Water volume is water volume, no matter the foot print, and water volume is just as important for turtles as it is for fish.
That is a seriously cool tank though. I would buy it in a heart beat. I agree on fiddlers, with an island in the middle, or maybe a marmorkreb or dwarf cray. But no turtles please. Especially not "turtles" in the plural sense!!!!