Phone [Q] A Weary Traveler [Questions About ROM Flashing]

First, and foremost, hello. I am new here, and I'm glad to be a part of the forum. Secondly, I'd like to apologize if I am posting this in the wrong section.

The topic I'd like to ask you a few questions about today is ROM Flashing, and the steps I should be taking to stay safe. I originally started playing around with Android devices around a year ago, but I recently started to delve deeper. I am the proud owner of an AT&T ZTE Z998 Mustang, and I have rooted it successfully. While in the process of rooting, I saw all of the warnings of how I should do a NANDROID backup, so that was my first instinct. I tried installing Clockworkmod, but to my dismay, they did not have an official version for my phone. I did happen to get lucky and find a version someone ported on their own. I attempted for a whole night before falling prey to error after error, and decided to give up. I went on without any backups, fiddling with my device. Luckily, I did not mess anything up, except for "accidentally" deleting the AT&T startup animation. (Haha.) Tonight, though, I happened to get the urge again, so I made some more attempts. After receiving the same error, I decided to look that error up word for word, and found that I did not have enough room on my Micro SD. I managed to clear enough room, and jumped in joy when I saw that I could delete an app, then restore and have it be back again. It was quite a big accomplishment for me. I've decided that I want to try flashing a custom ROM. The ROM I have chosen is the MIUI ROM. It seems quite popular, and I enjoy the iOS-esq feel of it, as well as its seamless theme changing. The only problem is, there is not an official release for my device. In fact, I have yet to find any ROM with an official release for my device. That is why I am coming to you guys.

My Questions:
Can I use a ROM that has not been specifically made for my device?
Will I still be connected to the AT&T Network?
If so, can I expect errors?
Am I truly safe with this newly acquired backup?
Anything you think I should know?

I'd like to thank you for any help in advance.