My off the cuff answer is
My off the cuff answer is “Try it and see”.
More considered view is that you will have a lot of vailing and visible edges of the glass pieces. Going to a very high temperature or with a long soak will not completely eliminate these, so you need to incorporate this into the “look” of the piece.
There is likely to be a significant amount of devitrification too.
Instead of making a mould, you can place batt washed dams to hold the glass to the size you want. You will need to do some thinking about the amount of glass you need to achieve a 6mm thick tile. This is where the metric system of measuring comes in handy.
a 10cm square tile (4″) 0.6cm thick requires a volume of 60 cubic cm of glass. Glass has a specific gravity of around 2.5. So you need about 150 gms of glass to fill the space.