The MP 650 heads will NOT fit. they are the same basic shape, but the nubs are ground off so it can´t be fixed in a MP 600.
All MP 600 series (blunt-, needlenose and EOD, Blunt and Needlenose with carbide cutters) are interchangeable.
The shear from a Paul Chen or Excalibur Multisnip will fit, too.
The Diesel head should gft with some fiddling.
MP 400/450 heads are smaller.
What is your plan, maybe we can do a trade?!