If I do the demonic initiation, will I no longer be able to work with entities, for example, from the Egyptian or Roman pantheon, or with angels? Or could they even try to harm me?
It depends on what you mean by initiation.
Generally, you can still work with angels when working with demons. In my experience, angels will sometimes recommend demons for certain tasks and vice versa. In one of his early books, Questing After Visions, EA Koetting talks about being advised by his spirit guides to work with angels after a particularly dark immersion in the demonic.
With Egyptian and Roman deities, it really comes down to the specific god and their mythology. If a certain god is known as a demon slayer, for example, then it’s probably a bad idea to continue with demons while working with them.
Egypt - Set and Horus spring to mind.
Greece - Perceus and Medusa spring to mind.
If you’re using Shem angels it could work strictly from the angel and emissaries supervising or micromanaging the demon which can be risky.
Solomonic magic on certain demons will absolutely go south.
I think DK is dead on.