you are correct about the non stretching of fireline. But 50 lb test is quite overkill for beach fishing. And if you are bait fishing you are much better off with mono

good that you switched. 40 lb mono is also pretty heavy depending on where you are fishing if there are any rocks and so forth. As you gain experience you may go down to 25 lb mono.
I use 14 lb fireline for plugging the beaches and it seems to work well for me I can feel the hits better.