What you are proposing feels like it could work and give you a stable surface from which to mould directly from, but it does feel like a hell of a lot of work to try and get your mould surface from

You'd also need to go quite thick on the fibreglass in order to give everything the stability it needs to withstand multiple pulls using vacuum. Also, when you're introducing filler and applying gel coat as the last stage, whilst relying up good mechanical bonds between materials, i think that there's a high probability of sections breaking away. Perhaps not straight away, but I think that the finished mould might not be as resilient as one made via the more conventional route

By all means, the materials you've suggested would likely help make a good pattern of the part you want to produce, but you'd likely get a much better mould capable of being used repeatedly by looking to use the pattern as a base from which to make a 'proper' mould from. It might take you a little more time overall, but it should be bound together much more reliably.