Is a Creatures from a realm always magical 
Is it posible to make a skeleton thats is not magica?l
Of course not, Demons are Infernal, Angels are Divine, and faeries are, well, Fae.
Depends on what you mean by "make". From the storyguide point of view of creature design, everything's possible. Through vanilla Hermetic Magic, you probably only can produce Magic creatures (though with enough botches...), while House Merinita's Animae Magic Mystery (Tour 
) will be able to create Faerie Skeletons withough much trouble.
There is no reason why every creature from a regio/aura HAS to have a might score. One of the beauties of this game is that the SG has the freedom to designed mobs the way they wish, and not rely on a Monster Manual. Certainly, powerful mobs should and almost need a might score, but trash mobs don't have to have one.
As far as the skeletons go, you need for game clarity the cause that made these creatures "animate". Was it a spell or item (prolly no might score). Was it the influence of the regio/aura/region? Maybe no might score -- all creatures that die in this place become animted undead UNLESS you do something special.
Make it up as you wish.. just try to have it make some sense.
My take:
Any "natural" mystical creature should have a might score, even as low as 1. This is because they're, well, mystical, not mundane. And even "real" creatures, like rhinoceros, could have might, due to the fact that, at the time, they are considered as mythical.
But there are some exception, like some skeletons: Skeletons animated by ReCo magic are not autonomous skeletons, and thus, have no might score. however, a skeleton that just rose without outside intervention, such as in an infernal place that raise the dead, should have might.
very low might makes it very easy to familiarize, migh be your cup of tea, might not
Hmm, this was actually the thing I had in mind. 
Or that the Skeletons was raised bye at powerful Demon.
I would like that the skeletons, could pass trough the Agies, and that is why that i don't want them to have at might score.
If the skellies are animated with a spell they do not hacve MR. They "die" if the spell is dispelled, though. Itr happens with the animated hermetic skellies as well.
In fact, they are not "creatures of a realm" at all. They are chunks of bone that move according to a pattern because there is a spell moving them around.
Cheers,
Xavi
What about Zombies, Vampieres, Ghosts or other Undead Creatures?
Another thing..
How high should at Might be if the skeleton i animated by an infernal aura or a Demon?
The obvious (and therefore probably useless) answer is that they have whatever might will make for the best story (keeping in mind that a lack of consistency tends to be particularly detrimental)
My first thought these critters that each skeleton is inhabited by a demonic spirit. So I'd create an appropriate little demon, give the demon a power that lets it inhabit corpses and move these possessed bodies by its will, and then use the demon's might.
Might 10 for easy critters like skeletons or simple ghosts
Might 20 for your nasty buggers: mummies and nasty skeletons
Might 30 for Cerberos
Might 40 for the skeleton king or a powerfull vampire
Might 50+ for elder vampires
Might 'you can't reach this' for Cain himself
Just an indication with some examples. As long as you have fun with the difficulty as indicated by the might scores.
IMS we've got a DEO-specialised Maga, so our infernal creatures are usually a bit stronger. Though I recently realised that swarming with loads of might 20 creatures might be a better idea: that lets the other magi play around some more aswell.