The most effective starting strategy for the Inferno faction.

I agree. If you spend a day building a Great building in your castle instead of another, higher-level dwelling or economic building, it should be very well justified (it's better to build Succubi than a Great building for Demons), otherwise, your development will be slowed down. As a rule, in such cases, a Great building for Scouts, Beasts, or Centaurs is justified... for Demons, it's only the Hellhounds...

You can't really rely solely on Cerberuses at the beginning. (unless you happen to find Groll in the tavern, that's a different story). In my opinion, you need to upgrade at least the Imps, as this is necessary to increase both damage and HP (I don't think I need to explain why it's important to keep as many Imps as possible alive). With Demons, I agree, you can wait a bit to build Nightmares or a Demoness instead.