Can you do a creo ritual to create something with a perdo requisite?
such as creo terrum (perdo) to create a stone with out(or with less) weight.
Is it possible to have a demonic power that is totally hidden from divine notice? Such as a corupted priest that can go unnoticed while walking into the vatican
These sound like the old medieval theological questions:
"How many angels can fit on the head of a pin?" or "Can God make a rock so heavy he can't lift it?" 
Somehow Creo with a Perdo req just sounds wrong. Maybe its just me though.
These actually have answers though... I'm not sure if this violates essential nature by creoing something and have it lack all the normal properties. Assuming it doesn't...I'm unsure if it would have a perdo requiste or just be a highly unnatural effect and increase level as needed.
I've spent so little time with the divine/infernal books I'm not even sure what part to search in to see if the second question is possible.
I... don't see why not. I'd say a creo effect creates a nearly perfect thing of its kind; you can create lesser versions for free, e.g. rotten meat, but to create things that are unnaturally imperfect, e.g. missing their weight, a perdo requisite does seem appropriate. A very interesting thought.
I too am not very familiar with the Divine and Infernal, but it is my impression that in cannon the answer would be "no". For my games, the pope himself may very well be a diabolist 
Probably, but it could be a Muto requisite, or alternatively require two spells.
No, the Divine is not deceived by the Infernal, and the many angels that protect the See of Peter would no doubt work to ensure that the corrupt priest was uncovered.
Perdo can remove some thing's weight while leaving the rest of the properties intact (Arm5 p78), but it also has to make something a worse example of the kind of thing that they are.
The spell to create an unnaturally low weight object might be terrifically difficult but it shouldn't require a muto requisite.
It depends. It can if the duration is NOT momentary. With momentary rituals you can create only natural things.
Which one fits better to your saga.
It is entirely possible, with Creo alone, to create something damaged - but not with its intrinsic characteristics changed, that would indeed require either a Muto or Perdo requisite, depending on what kind of alteration one intended.
Less weight is the loss of a characteristic, not the transformation to something else and is therefore a Perdo requisite indeed.
Quite an interesting idea.
I would have to check on the duration however...
As for the second question, the Divine can never be fooled by the Infernal, that is true. But if it can in your saga... more power to them? 
Is a weightless stone a worse stone?
If yes, it is Perdo requisite
If no, it is Muto requisite.
Cheers,
Xavi
Conceivably, one could create stone that weighed considerably less than a normal rock, because such naturally exists, and is known in period; Verdi covenant would definitely know about pumice, because the covenant is near to Mt. Aetna... There are a whole range of spells that could improve pumice's characteristics (all of them Muto - put them into a constant effect item) - you could make a boat of stone with the stuff.
However, since all things have weight, a weightless object would be considered "Highly Unnatural" and thus, require a Muto requisite to be weightless.
Steve
It shouldn't require a Mu requisite - just a higher level (you're not changing anything, just making a more complex versjon of it...)
For the same reason no Pe requisite...
I agree with Ulf here...higher level, no requisites...
But you cannot, with Creo alone, create a winged green horse, because such a thing is unnatural, that is, doesn't exist in the "mundane world".
So unless one was creating pumice or some other kind of light weight stone, it would not be possible to create it with Creo alone; either a Perdo or Muto requisite would be necessary.
Which exactly is still being discussed, and I have to say I'm pending on Perdo, since it does, as quoted, remove something's weight without removing it's other properties.
Weightless stone other than pumice is not natural stone. Non natural stone requires Muto.
Unless you consider that a weightless stone is a worse version of a stone. If your answer is "yes", then uit is a perdo requisite. Easy
Cheers,
Xavi
Or you could add a Vim requisite, to create a magical stone with no weight.
I disagree. Adding a vim requisite does not get around the restriction that only natural things can be permanently created.Also weightlessness is specifically covered in the description of perdo. A muto requisite for weightless stone's unnaturalness is not necessary.
You can create magical creatures by Creo-Animal (Vim), for creatures which cannot be created using "Normal" creo magics.
If I wanted to create a stone, which magically had no weight, I could create one using a Vim req. Same as if I wanted a stone which had some magical property, I would use CreTe(Vi). In this case, the magic is that the stone has no weight.
The difference is that magical creatures do exist naturally in the setting. If weightless rocks exist in your setting naturally then I suppose that creating one with a creo ritual is fine.
However I would susspect that there is no place in mythic Europe that you could to find naturally occuring rocks that have no weight.
You can CrAn(Vi) yourself a real gryphon with a momentary creo ritual beccause int he setting there are actually gryphons, but you can't CrAn(Vi) yourself a real crumple horned snorkack with a momentary creo ritual because in the setting there are no real crumple horned snorkacks (your mythic europe may vary).
Ooooh but I WANT one!!!!!
Hmm, that is a good argument Erik. I was looking at it from a different perspective, but your idea makes a lot of sense.