50 classTypically (at least at IRCHA), the classes are set up to regulate engine displacement.
Most of the pilots flying 50's are running either 570mm or 600mm blades as there are lots of blade choices out there.
Putting larger blades on a drag race heli is not really a good idea unless you have an abundence of power and need the additional surface area to "put it to the air".
I have run an Xcell head on my drag race machine in both the 30 and the 50 class with anywhere from 550mm to 600mm blades.
So, in answer to your questions,
1) Can you run the Xcell head - Yes
2) As long as some part of the heli is on the ground, you are good to go. - Also leaning forward is OK.
3) There have been no sound DB numbers enforced at most races, but with the modern technology, most pilots are running some type of muffled pipe.
That is at IRCHA, typically all fields including the AMA sites have some type of noise restrictions.
Just FYI, the old Muscle Pipe 1 for a 60 works pretty well on most 50's
Hope this helps,