Depends on the size of your room and on how loud you want the SPL to be at the center listening location.
If this is for a home theater system that uses a consumer (high end) surround receiver that has un-amplified outputs and will be used for watching movies. Modern surround receivers have a built in RTA Mic and balancing system that will ping the room and tune your surround system automatically. Then it's just a matter of adjusting the trim output to your amplified speakers.
If this is for a pro surround setup for studio mixing and mastering / post, you'll need to balance the room by hand with an SPL meter http://www.audiogurus.com/learn/speakers/calibrate-surround-levels/272
Also suggest reading this: http://tweakheadz.com/surround-sound-studio/
To answer your question more directly, your center speaker is meant for speech and is not meant to carry any LFE signals. Because the E66 is meant to sit flat it is the most convenient choice. Eris 8's (or 6.5) will work (as most LFE will be on the sub, not the L/R speakers (also depends on the size of your room). While E5 make ideal Rear Left and Rear Right and a T10 subwoofer for the .1.
I would suggest posting this question in the community forum the user community may be able to give you the most informed answer for surround setups.