RE: Is this a dumb idea??
I do not think you are going to be happy using all of those components on a T-Rex.
I do not know which brand of servos you are using. Do you have more information that 8 gram? Most people agree that for the T-Rex 450, you want a very good cyclic servo. Hitec HS65 (or HS56) is the norm.
I believe that the SA comes with a motor and ESC. It is pretty well matched to the requirements of the T-Rex, so you would be well off to use them.
I do not know anyone using a 3s 1500mAh battery for their T-Rex. 2000-2100mAh is the norm.
What gyro do you have? A decent head hold gyro is a huge plus. Futaba GY401 is the norm.
The 450X and XL are older models that have had missed out on many improvements tht went in to the newer kits. SA or SE would be a better choice.
Vince