I have that Tunze nano osmolator. You can actually program it to pump at different rates depending upon the tank size. It has an auto-off feature that stops the pump if it has been pumping for more than the allotted time (can't remember the exact number...but something like 2 minutes). Mine is setup so that it can't add more than 1/2 gal to my tank before shutting off.
It'd probably still overflow a 2gal in a hurry...but I'm not sure there are many options to prevent that other than running a controller...and who wants to do that on a 2gal nano build.
I've had no troubles with the Tunze...it's been running flawlessly since setting it up ~2 months ago.