Your project seems similar to the project in Bullseye's TipSheet 3: Working Deep
, and the instructions there address the issue of stacking sheets on which frits or powders have been pre-fired. You will find suggestions for helping the bubbles escape, both through the glass layup (arrangement) and the firing cycle.
Download the pdf file at this link: http://www.bullseyeglass.com/pdf/techno ... eet_03.pdf
The TipSheet includes some recommended fusing cycles. You could start with these cycles, take notes on your results for each firing, and come up with cycles that work very well for your particular kiln and project.