Don't worry about all of those details.

Just flash whatever ROM you want on your phone to begin with.

I suggest Ivan's Eris_Official ROM.

Eris_Official 1.0 Alpha With Controllable Overclocking! [5/9/2010] - xda-developers

THEN, use my Cricket thread. All you need is to flash the eris mms and web .zip file.

So, your phone is rooted. Hopefully Recovery is installed. If not, go to the first page of the All Things Root forum, and follow step II or III (whichever one involves installing Recovery. it's necessary.)

Download Ivan's Eris_Official I posted a link to above, as well as the Cricket mmsandweb .zip file I posted in my Cricket thread: Multiupload.com - upload your files to multiple file hosting sites!

Put both of those .zip files onto your SD card (not in any particular folder, just drag them right onto the SD card. If you need help with that, reply asking for help).

Boot into Recovery (turn off your phone, then use Volume UP + Power to turn it on). Do a Backup > nand backup (will back up your current phone configuration).

Then use VOL DOWN to go the previous menu, go to Wipe > Factory/Data Reset, then Wipe Dalvik Cache.

Then press VOL DOWN again to go to the previous menu, go to Flash Zip from SD Card, and flash Ivan's Eris_Official .zip file that is on your SD card. Then when that's done, once again, go Flash Zip from SD Card, and flash the cricketmmsweb .zip file that I posted a link to above (it will overwrite the right files, including that u2nl that you have, where they're supposed to go, AND it wil get your pix msgs working).

Then once it's done flashing, reboot phone. If all the PST settings are in there (aka when you open the dialer and do ##778, if all the settings are correct), then your phone is now FULLY 100% on Cricket.

Reply with any troubles, or see my thread:
http://androidforums.com/all-things-root-eris/69613-eris-fully-cricket-root-reqd.html

 

So put the rom in bin then flash?

 

I'm having problems trying to get the info from cricket as their phone support isn't live