i dont have any experience with them but GTP are known to be nippy even as adults and they are fairly challenging to keep as far as husbandry goes. they are more of a display snake that shouldnt be handled frequently.
you said you already have a red tail. how long is it now? if its an adult and you still want a larger snake, i would throw in my vote for a MALE burmese python. they will grow to be 10'-14' and are known for being extremely docile snakes. if your red tail is not yet an adult, you may want to wait until it is and then make your decision as to whether you still want a larger snake or not. hope this helps