You'll need 250 hours PIC in airplanes, of which 100 is X-country and 25 night. All the other time requirements should be covered by your helicopter ATP. To add an airplane category rating to an ATP you'll need to pass another FAA written. There is the ARA test to add single engine on to your helicopter ATP. In 2016 it had a 50% pass ratio. Two people took it. Or maybe one person twice. And you'll need to pass an ATP checkride.
I don't know why PI is saying you'll need the instrument airplane rating. That would be for someone who doesn't hold an ATP already. The ATP checkride is outlined in the PTS and the same as an IR ride but to a higher standard. If you go the commercial/IR route there are two checkrides. The commercial checkride is a bunch of visual maneuvers you'll never do again and then you have to IR checkride.