It may work depending on the batts you buy for the rex but the rex batt will be well over 800mAh and charge slow on the eflite charger (slow charging=[&o])....
however at the cost of setting up a Trex I think the cost of a new charger will be a very small % of your total investment.
a trex is $200 to $500
radio equip is $200 and way up
gyro is $50 to $200
bl and speedcontroler is $100 and up
batts and $75 to $100+
I believe most people wind up spending around $1000 for a nice trex setup.... a good $50 to $150 charger would be a good investment to go with such a nice bird.