can I flash the radio .27 or .98 on lollipop ?
Yes... I assume you mean the .27/.98 hybrid radio. Although I haven't heard anyone say it is flawless or not.
Try the .27/1.04 or .33/1.04 hybrids, if neither work an older version won't either.
Honestly, the 1.04 hybrids are the best ones I've used for WiFi and battery friendliness. All .27 based hybrids will perform the same with mobile data, as will all .33 based hybrids, since essentially what the hybrid radio is taking the stock version like 1.04 and removing the mobile voice and data radio portion and replacing it with that piece from the old JellyBean radios where LTE wasn't disabled yet, the all use those same radios as the base.
You are welcome to try older radios in Lollipop, they may work fine, may not work at all, or give you anything in between... Radio/firmware mismatches often result in rotation, camera, and other "sensor" related issues, but you can't really brick a Nexus device by flashing an incorrect version of radio.