I have dark brown hair, and I'd like to have red and blonde highlights.
I planned on using a cap for the blonde, doing every-other hole, for thin streaks.
Then I thought about doing the red by hand and using foil, for a little bit thicker streaks.
I'd like the colors to be random, and not too semetrical. Kind of messy, I guess.
But I don't want the colors to look weird if they get mixed...
So my question is, what is the risk of the red possibly going over the blonde?

You'll find that the red will "mix" with the dark brown, or with the blonde. When it mixes with the dark brown, all you'll notice is a little reddish tint on the brown. When it mixes with the blonde highlights, you'll have red or orange (depending on the shade of red that you use). It won't look all that great.

Hair coloring is transparent. It's not like paint that covers whatever's underneath it. Haircoloring shows what's under it.