Yep, you're right that you need a 7 channel receiver. I forgot that 7CH rx's are rare in the micro class. You can still use channel 5 just fine, and use the "gyro" menu in the transmitter to adjust gyro sensitivity. In this case, anything set under 50 will be rate mode, anything over 50 will be HH mode.
The only downside is, like I said, the above mentioned bug when switching between flight modes. I don't know of any way to fix it, yet.
It works just fine in normal mode, but you'll find it rather annoying once you move to idle up flight.

That, and you don't get as fine adjustment when you've only got 50 -> 100 to work with vs the +/- 100 you get with EPA.

I don't know of any setup guides off the top of my head for the Eclipse 7 and the T-rex. My best advice would be to have a look at the basic setup of how the electronics should work on the T-rex, even if it's for other transmitters, then cross reference that info with your Eclipse 7 manual to configure it properly.

This might be of some use to you:
http://www.trextuning.com/setuptrex.php