If we do not count in the Power Focus then can I still cast at Force 7 or 8? It's only available due to the Power Focus infusing the Magic attribute with additional power (making his Magic an 8 instead of a 6). I suppose what I'm seeing is that the Power Focus, as such, does not inherently affect your spells it affects your Magic attribute and per the guidelines affects anything that uses the Magic attribute. Hence it has no affect on Drain, for example (I read some previous post here where someone mentioned trying to use the Power Focus on Drain Tests).
I know Max already addressed this briefly, but it's worth pairing it with the OP's question and being specific:
A power focus adds its Force to all tests in which the magician's Magic is included.
Unless otherwise stated, any modifier mentioned is considered to be a dice pool modifier as noted above.
These two sentences, taken together, state that the dice you get from your power focus are a
dice pool modifier vice an
attribute modifier. Therefore, they do not increase your magic attribute, only your dice pool. So in the example in the OP's question:
Magic = 6,
Max Force = 12,
Force to not be Overcasting = 6
and, as previously mentioned:
Book is actually very clear about this subject
Regardless of the number of foci a magician may possess, only one
focus may add its Force to any single dice pool.
So no you can not use both a power foci and spellcasting foci to cast the same spell, only one or the other.
Therefore, final DP is: Magic 6 + Spellcasting* 6 + Combat Focus 3 = 15
or, for a non-combat spell:
Magic 6 + Spellcasting 6 + Power Focus 2 = 14
*Original post doesn't specifiy this, but the math in the original post does:
For demonstrative purposes. A Magician has a Power Focus 2 adding to his Magic 6 giving him an effective Magic 8. The Mage then casts a Combat Spell using his Combat Spellcasting Focus 3. Would his dice pool would be 8 + 6 + 3 = 17? Or does this crossover the "Using a single focus" rule?
Emphasis mine