Maybe you misunderstand what a cereal mash is.
Grains that aren't malted need to have their starches gelatinized so that the amylase can get to them.
Flaked grains are steamed and squished into flakes, then dried. So, they are pre-gelatinized and then dried.
So the idea is that you crush those non-malted grains and throw them in the rice cooker with as much water as you feel like and cook 'em until they are mushy, and then add them to your mash. Or add them to your strike water and then add that to your mash.
It can be tricky not to overshoot your water volume doing that. You'd have to weigh dry vs. cooked to determine how much water is in it. But that's only if you're worried about your exact volume.
If you consider that most specialty grains provide no enzymes, you can typically get away with 20-30% specialty grains and adjuncts before you need to worry about using 6-row or pilsner. With pilsner malt as your base you can go to 40%. With 6-row you can go as high as 50%, I think, but i could be wrong on that.
Edit: Wait, no, pilsner malt has LESS enzymes than 2-row. Sorry.