I've used acrylic artist's ink, GW's old inks and GW washes over Humbrol enamels in the past, and they all worked just fine.
One GW shopdrone did try to tell me their inks would bead up and run off enamel paints, though. Thankfully but unsurprisingly, he was dead wrong.
I actually just retired my last active tin of Humbrols to the big paint rack in the sky, as I finally got around to buying Reaper's metallic gold triad of paints. Nice not to have to keep thinners around on the painting bench anymore.