Rainbows don't normally eat their eggs if they are well fed.
Most P. signifer lay eggs in the substrate but some lay in plants. If you move a gravel cleaner over the substrate and collect the water in a white bucket, you might be able to see eggs after the water settles. These can be put into a hatching tank and grown up.
Never new they lay in substrate, I will look at this I think. I do have the cories so maybe they might be eating them if they are spawning in the substrate, do you think?