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?
I try and set up "nest" to keep the eggs safe. I had red eyed tetra fry survive to adult hood (14) when my 55 gallon tank was my LOTR castle tank, there were plenty of places for eggs and fry to survive, with my new set up I have had no fry so far.