You can use any of the radio's available on geekfor.me including the stock 1.5 radio. Regardless of the rom, use the radio that gives you the best signal. Personally, I get the best signal in my area with one of the radios that was released for fresh 2.0b. I've tested them all out including the VZW radio that was released by flipz. Check out your signal strength (dBm) by hitting menu> settings > about phone> network. A great signal is anything in the -70's, even low -80s is pretty good for some people (the closer to 0 your dBm is the better your signal. Some phones measure in positive numbers while others are measured in negatives.
***Make sure you don't accidentally flash a GSM radio from the wrong forum on a random site or xda. It would be best to stick to the radios provided by flipz on his site
www.geekfor.me***
As far as going back to stock, I am pretty sure that flashing a stock signed RUU will not change your radio. Either way before you go back to 1.5 change your radio back to stock and then run the program to begin the stock flashing process. So yes it is possible to go back to stock.
As always make a nandroid before attempting anything. Good Luck.