Hello,
Can we use the animations provided from epic with the free characters provided from Mixamo?
When you re-rigg the characters from mixamo you will be abel to use the anims from the starter pack ^^ otherwise it is not possible, because it is using another rigg 
ah ok, thnx.
Is there a tuto (if i can do this) of how to rig a character with an existing skeleton?
Actually you just have to watch an ordinary rigging tutorial (then you learn how to rigg + you will know how to use an existing rigg). But the main part of your process will be to set up the vertex groups -> so that every bone is bound to the right vertices
Probably this tutorial will help you: ?v=4qWdmYowNHM 
But make sure that you fit the mesh to the rigg and not vice versa!
Easiest way I’ve found for switching skeletons is the following (works for 3D Studio but could probably be transferred to other applications as well):
Import the character and select the “skin” and then press “Edit Envelopes”. After that click “Save” and save out the envelopes as a separate file.
After you do this, remove the skin and freeze the bones so you don’t select the wrong ones.
Then export the TPP skeletal mesh from UE4 and import it. You might want to scale the whole thing to a fair match with your character, then delete the skin and the mesh, leaving only the bones.
Move each bone one by one, starting with the highest in hierarchy like the root and then hip/pelvis and so on, so they matches the location of your initial rig.
When all of them are in the same location as their respective counterparts, select the mesh, make a skin modifier and add all the bones using the “Add” button.
After this you just click “Load” and load in the envelopes that you saved earlier, arrange the right side to match the left side (you might want to start up another 3ds to compare bone names, they won’t be exactly the same). Use “Blank” to mark bones that you don’t have a counterpart for and then it should be done.
You might have to tweak some parts by hand, for example the TPP skeleton doesn’t have the same bone setup for feet as Mixamo, leaving toes withouht skin weight when I did this.
ok thnx, but does the character have to be in the same size of the TPP skeleton or i can skale it?
You should look at the avatar default pose, when you select the avatar in press on configure you character should be in T stance. He should be Z facing and Y-up, both arm parallel with X axis. Also his feet should touch the ground.
Mecanim is using this default pose to calibrate the avatar, if this step is not done you will get unexpected result when you retarget your animation to this character.