Quick query that maybe someone else has already encountered.
Magus A creos a big rock and then uses a seperate spell to rego it at magus B's head.
Which spell needs to penetrate? Creo big rock or Rego big rock? Or both?
I'd say both.
To affect someone with something created with magic (as a creo ignem spell affecting directly a magus) you need to penetrate. A rego spell would also need to penetrate if you direct the created thing at the target (if the rego doesn't penetrate the rock will stop in front of the magus).
If you ritually create the rock(using vis,instant effect, etc.), the rock become natural and only the rego spell need to penetrate (if you still want to hurl it at another magus).
Both have to penetrate yes, which makes it tricky against magi.. but then, I usually then count damage from both spells to make up for it, because you're not only crushing them with a heavy chunk of stone, but you're adding speed to the matter.
Of course, if you wanted to be more sure of penetration, you can just use aiming and drop it on then head with gravity, but then you're only getting the damage from the crushing weight of the rock, not any additional from the ReTe
both need to penetrate.
I would also say both. This is why creating the big rock in mid air and allowing it to drop on the enemy magus is much more efficient. A single spell with Mom dutration instead of 2 separate spells. Still needs to penetrate, but it is one spell instead of 2.
Plan B is to take a nearby bolder and toss it at the mage. Boulders seem to be scarce when you need to fling them around, though. But similar ReTe-able items are usually nearby. It also needs to penetrate. Again it is a single spell, though 
Cheers,
Xavi
Both.
Which is why it ideally is important to note the penetration of most spells that have a duration beyond Mom if in a a situation that might offer MRs.
both
so work on your targeting and rego the already present rock/tree/horse up and see what happens when it falls
Of course the game breaker plan is to target the rocks underneath the the victim and then teleport them over the top of the vitim. You'd need to make a targeting roll, but you wouldn't need to penetrate MR.
Funny you should mention that, one of my players has a spell that does just that. He doesn't target stone, just a very large amount of dirt. His target is hence dropped down a deep hole, crushed by the falling dirt and left trapped buried quite a distance down under a few tons of dirt.
Game breaking? it does make killing enemy magi rather easy. If they are standing on dirt of course.
My original idea for asking was that i wanted to summon a swarm of flying human teeth with one spell (CrCo (Re)), then have a bunch of rego spells to compel this swarm to do all kinds of fun things, such as attack someone, swirl around in a defensive cloud, attack a group, etc.
Thus i would need to penetrate with both the original spell and the rego spells i would use to compel the teeth. I could of course use a muto vim spell to change the effect of the rego on the original spell. But then i can't stick the effect in my talisman (without the consumate talisman virtue, which i don't have). Would the MuVi spell need to penetrate also? since it is only trying to change the rego nature of the original?
I admit that I am guilty of hyperbole.