I know when using modal interchange I can borrow chords from any of the 7 parallel modes of the major scale. But I'm wondering if it's okay to use chords from scales other than the standard 7 modes.
For example. If I'm writing a song in the key of C major, can I borrow chords from C harmonic minor, C phrygian dominant, or C minor pentatonic?
Side note: The most common (one might even say "signature") chord pointing toward harmonic minor is the V (or V7). However, because the 7th degree of the scale is raised in harmonic minor, just as it is in major, the V chord already exists diatonically in major. So you could borrow chords from harmonic minor, but if you wanted to specifically use chords implying harmonic minor, it's more difficult. Of course, why would you ever restrict yourself in this way? Borrowing from all kinds of minor is really common.
You CAN actually do whatever you like in music! If whatever you like sounds good (even just to you), then yes, it's permissable. Not many musicians go to the firing squad for playing what they want, regardless - although there's an idea there somewhere!
Seriously, by the time you've counted all the notes from the parallel scales and modes, there's none left. They all feature somewhere! The most common borrowing is major/minor, which actually gives you C D E♭ E F G A♭ A B♭ and B. In fact, the only missing notes are just the two - C♯ and F♯. And if you include C blues scale that's F♯ catered for! Oh, and the missing C♯ comes out to play when tts is used. There you are - all of them.
As I say to students at some point - even though you know all the scales, you can use any note anywhere at any time in any piece. It's just being cute enough to know where it'll fit best. But it will fit somewhere!
Incidentally, using pentatonic minor notes is no different - they're already covered in the minor scale notes.
Yes, it's pretty common to borrow chords from the minor scale with the same name (parallel key). A really common progression is instead of playing IV to play iv, which is borrowed from the minor scale.
Generally, you can borrow any chord from the parallel key of your scale. Some chords, like the iv,bvi,viio are more common borrowed chords, and used more often than others.
Wikipedia has some good examples from songs if you are interested
