You can't use firmware for another model phone. That's modding 101.
You can root any ROM (well, almost - some phones are locked down so tight that it takes a while for someone to figure out how to root it) but ...
1) It has to be the ROM installed in your phone, and it has to work in your phone. Rooting a ROM for another phone won't make it work in your phone.
2) Unless you have a good reason (being able to say "my phone is rooted" isn't a good reason) to root a phone, the possible costs far outweigh the advantages.
The ONLY thing rooting a phone will do is allow you to run apps that require root. It WILL NOT do anything else (except void the warranty). So unless you have a need, or an urgent desire, to run an app that requires root, there's no reason to root the phone.
(About the only semi-legitimate reason for most users to root a phone is to use Titanium Backup to back up your apps and data. But only semi-legitimate, because you can back them up without rooting and just a tiny bit more work.) People have been running Windows for years without doing the equivalent of rooting (and it's right there in the right-click menu), and without missing anything or having any problems. Most cellphone users don't know what rooting is and have never rooted a phone, and never have a problem.
The rule of thumb is that if you can't just run adb and push root to the phone (using an exploit), without needing to be told how to do it, you don't know enough about how cellphones work to root yours. (One wrong command and you have a paper weight, not a phone.) There's a reason experienced system administrators almost never log in as root. It's called "safety". Tell a *nix computer (which is what an Android phone is) to destroy itself, without root, and not much happens. Do it when you're rooted and you'll be spending long hours into the night getting your phone to do anything more than discharge the battery. Even if you don't realize that's what your command will do, if you're rooted the phone will calmly proceed to commit suicide.
There's a difference between a homeowner who grows a few vegetables during the summer and a farmer who farms 5,000 acres. There's a difference between the casual cellphone user and the technically-educated person who rewrites the ROM because he doesn't like the way it works. (There's nothing wrong with being a homeowner who buys 3 tomato plants from Lowes every year - but don't spray arsenic all over the place to kill bugs on a windy day, unless you like living in a totally unpopulated neighborhood.)
