Yes and yes. I have used a Heil mike on my Flex 6500 since day one. Play around with the tx equalisation to get the sound that you like best out of your Heil mike, then set that as one of your transmit profiles so each time you use it, you don't have to reinvent the wheel, just engage that profile.
It will work fine, I have used the ICM on my Flex with no problems once your rewire the mic plug. In the main settings menu, radio menu, Phone/CW, turn off +20DB and enable BIAS. Use the mike equalization for best sound.