Congratulations on the successful root

. So, you actually flashed the new radios along with the ROM, correct (they are separate downloads)? Assuming you did, you can now flash any 3.70-based ROM without having to worry about your radios. You only have to update once (until we get a new update, obviously).
As for other ROMs, here's the whole list:
HTC Supersonic/ROMs - XDA-Developers
You can flash any 3.70 ROM you want (as I said before). If you want anything older than that (3.30 and under), you must downgrade both the radio and PRI (or just the PRI, and flash a different kernel, but let's not make this difficult since you're new to it). Just stick with a 3.70 ROM. Most of them are getting updated or already are.
Here's a thread from here with some good suggestions and opinions:
http://androidforums.com/evo-4g-all-things-root/250161-poll-vote-your-favorite-rom.html
As for kernels, if you go back to the myn's warm xda page, you can see that he has some recommended kernels in the OP (or first couple of posts). Basically, netarchy, ziggy's, kings, etc. are some of the popular ones. Read about what they offer before flashing anything. Of course, ask questions if you need. What specifically do you want to do with a custom kernel? Do you understand what they offer and how they differ from HTC stock kernels?