The FD-6400 was originally meant for 7-speed, but, I've never had any issues mixing 7 and 8 speed components. The problems occur when you move up to 9/10 speed. The chain you should use is an 8-speed, which is nearly the same width as the 7-speed, which is the determining factor.

BTW, I don't understand why you're adjusting anything while you're in the smallest cog/chainring combination front and rear. That's technically an "illegal" gear combination in 8-speed systems, you should get unpredictable results since the chain is really way far off the limits of chainline goodness.