Hi,
I created a skull by starting with low poly. Then in parallel I started sculpting a mouth on a sqhere with sculptris pro. The mouth looked good at least for a sculpting beginner like me. So now I wanted to melt the mouth with the scull. But that seems to be difficult if there are tools of different mesh sizes. Is there a way to melt these tools in a way that the edges of scull and mouth are really connected and it looks like the mouth is part of the skull?
You can merge the meshes into the same tool and Dynamesh them together, or use Live Boolean. Position the mouth carefully so it is poking up through the mesh, not sitting on top of it.
It’s always better to simply sculpt the subject as one though. This will produce better looking results than a character that has been “Frankenstein-ed” together.
Ah ok i tryed it once with dynamesh but I did exacly the wrong. The mouth was sittting on top and so it did not melt together. Ok I think you are right. But it’s a pitty . Now that the mouth looks good I have to sculpt it again. But I think live of an artist is a bit like that until you reach a profession. 
You might find VDM brushes of use for your workflow: