Evocation uses Power and control. Thaumaturgy uses Complexity and Control. Therefore evo-thaum uses evocation foci because complexity won't boost an evocation spell - only power does that.
Not necessarily. Evothaum might use special evothaum foci. Or it might not use any foci at all.
The rules really don't mention this issue. There are no RAW.
Anyway, I know from personal experience that using evocation foci for evothaum can be problematic. So I'd be inclined to use thaumaturgy foci, or perhaps special evothaum foci. (Complexity would count as Power, if using thaumaturgy foci.)
I'm not sure whether or not that counts as a houserule. Doesn't matter much, really...it's mostly just a question of how you define the word houserule.