MOST roms are interchangeable between the Avail/Merit.
As far as hardware goes, they are identical. The issues arise with the network the phone was designed to connect with. Avail Z990 for AT&T, Merit Z990G for Straight Talk, and Merit Z990G for Net10 DO have some 'stock' firmware differences.
Those mostly become appearent when you tryto root them. The Avail is super easy (T-Card and *code). The ST Merit can flash a update.zip, which will allow you to flash a rooted ROM. The Net10 closes that loophole, and you have to jcase ADB it.
After all that rigamarole, you get into carrier specific oddities, such as SMS, MMS, and other things. While MOST roms will work splendidly as an Avail OR as a Merit, you can (possibly, not probably) run into issues like that.
All in all, the best thing (which sounds odd) is to go ahead and try them, and see if they work on your phone, with your carrier. Just make a nandroid backup first, just in case the odd dragon pops his head out of the cave. If that happens, a restore is REAL easy. (Thus why I worked and worked at getting a 100% working CWM for this phone)
So, in conclusion, the hardware is the same. The firmware is slightly different. The carrier-specific setups can get wonky on you.
Basically, try the rom. If it works, great. If it doesn't...