I'm pretty lax about those rigid timelines of when to give what. If it were me, I'd do whole cows milk. Others can judge all they want on that.
Otherwise you could mix in water (or small amount of pumped milk) and some yogurt, whatever ratio gives a decent texture. That's what our DCP does.
I've also made it with pureed fruit that turned out waaaay too soupy. Doesn't really have protein, but it was a good way to use up a giant batch of super liquidy apples.