900 would be nicer but you could definitely use the 600s.
With the 600 you will get shady areas on the ends if you put the light in the centre.
I have the same on a 900 wide tank. I have only Anubias and Crypts and have no issues. http://www.fnzas.org.nz/fishroom/viewtopic.php?f=2&t=51675 this grows like crazy so it cant be all bad.
BTW, the only stupid question is the one you dont ask.