DD is 18m and we lowered her cot a few months ago so that she could climb in/out etc. She loves it.
We're moving to a rented house next month that is fully furnished, with a double bed in each room. I'm considering just buying bed rails and letting her have the small double (I think it's actually a 1.5 sort of size, do they exist?).
Yes, 1.5 beds do exist. They are called semi-doubles.
I would have no problem putting a toddler in a double bed. I moved both of my DCs to normal beds before they turned 2. A semi-double is wider than a single, so there must be less chance of her falling out. Just make sure that the bed isn't too high. I wouldn't recommend it if the bed is higher than standard. You might be able to take the legs off it if you are worried about the height.
Personally, I never used bed rails. I thought about it, but decided that I'd prefer them to learn to sleep without falling out so that they could sleep in hotels and other people's houses without me having to worry about bed rails there as well.
Instead, I put them down to sleep with their cot matress on the floor beside the bed to give them a soft landing if they happened to fall out. This worked fine. DD fell out about 4 times and then learned not to. DS fell out about 15 times and then learned not to.