I think having towers as surrounds depends on how your furniture is laid out. Since the E55Ti's are taller than E5Ti's, they may work better as the surrounds since the tweeter would be closer to ear level.
If you have another pair of speakers now, hook them up, set the E5i's as surrounds and see how it sounds. If it works and you're happy with the E5Ti's, they are on sale now for $250.00 pr. That's a few hundred less than another pair of E55Ti's.
My E5Ti's should be delivered today.
Good luck.