Can ISQL Perform forms co-exist with I4GL forms within I4GL and access the same SE or IDS engine?
Yes
There are some caveats, but the general answer is Yes.
The main caveat is that ISQL forms can have multiple screen layouts which I4GL forms cannot; I4GL limits you to one screen layout per form file.
The next caveat is that the ISQL INSTRUCTIONS section can contain information that I4GL cannot use - BEFORE EDITADD and such like are not understood by I4GL. The 'MASTER OF' information is not relevant to I4GL either, any more than the screen records information from an I4GL form is relevant to an ISQL form. Joins and verify joins and lookups are relevant in ISQL and not in I4GL.
The tag DISPLAYONLY in ISQL is changed to FORMONLY in I4GL.
So, the typical ISQL form cannot be used by I4GL, nor vice versa, but the core 'language' describing the forms is the same. You can certainly have an ISQL form running in one window and an I4GL form running in another window and they can both access the same database, and usually can access the same table though you have to be somewhat aware of locking issues.
There is nothing to stop you having your I4GL forms and ISQL forms all stored in the same directory - except, perhaps, common sense. Your ISQL users shouldn't have to pick through the set of I4GL forms as well as the ISQL forms - so I would keep them separate so that the ISQL users only see ISQL form files (and the I4GL users don't see any of the form files as files in file lists - though I4GL programmers might see them).