Movement of glued pieces
You comment that your pieces sometimes move in the firing although glued together.
Glues burn off a lot lower than the temperature at which the glass begins to stick together. So if you are glueing overhanging pieces, for example, they can move after the glue has burned off. My view is that if you are assembling a piece that will not stay in place while you are assembling it, glue will not help get the final result you want. If you are glueing to keep things stable while you move it to the kiln, you may find everything is ok.
However, glue tends to boil off if the temperature is raised too fast. During this process, the effect of the boiling will move the glass pieces that are most unstable.
Another way to deal with unstable pieces, is to make a large piece and then cut it up after fusing in to the sizes you want. Clean the pieces very well, and then fire them again to at least fire polish.
Glue most often leads to problems. In my view, of course.