depending on usage scenarios, a dedicated account might be the better option for Delta Chat. mixed usage might get tricky, even though Delta Chat tries to sort "chat messages" to a special folder
Also, end-to-end-encryption is not related directly to IMAP.
The issue with Hey/Protonmail/Tutanota is that they do not offer IMAP- or other standard-protocol-support - so you can just not log in with Delta Chat (nor with any other e-mail-client) to your Hey/Protonmail/Tutanota account.
If you log in to deltachat say with Posteo, you can write to other Hey/Protonmail/Tutanota accounts - but as the recipient cannot use Delta or other Autocrypt capable clients (Enigmail, K9), things cannot be encrypted. If Hey/Protonmail/Tutanota would support Autocrypt in their webclients, of course, this can change.