L.
We transitioned my daughter to a toddler bed at 19-1/2 months to free up the crib for the new baby. Since it was a toddler bed, it didn't have complete side rails (actually, ours doesn't have ANY rails) and twin rails won't work. We also have hardwood floors...hmmm...we just put a thick pad under her bed and I think she maybe fell out twice. Actually, I think both times were much later AFTER we decided to stop using the pad! We had a queen featherbed we weren't using, but any thick comforter would work to cushion the fall.
We did put up a gate at her door and that worked great. I think using a toddler bed is a nice transition for them because they don't feel "swallowed up" by the big bed. Also, if you are still using bumpers on the crib, it may help to remove them so your child is a little more used to an "open" feeling.