No way in hell would any of them can be inferior to Imperialdramon. And wasn't Megidramon Guilmon's Mega form? The one who got his ass handed by Beelzemon?Together they definitely would have been able to do some damage, but I'm just talking about one on one.
Seraphimon is just an average Angel type mega; his power is less than someone like Dominimon or Goldramon, and Magnadramon is actually the weakest of the 4 Great Dragon Digimon (strongest being Megidramon by far.)
One on one they'd be crushed by Malomyotismon...