1) You can install software from a different carrier on the same model. T-Mobile 7290 code will work on Cingular 7290.
2) You can not install different code on different models - 7290 code will not work on a 7520.
3) Nextel uses iDen radios. Cingular/T-Mobile use GSM/GPRS. Verizon/Sprint use CDMA. The software doesn't change that. And rule 2) above still applies.
I hope you didn't spend too much time on that thought