We've always had a king size bed and I have a TON of pillows. Once they get to where they can roll and have good head control and are napping in the bed then I just put pillows on the sides as a buffer. When they get to crawling stage though I put them to nap in a swing, then a crib or playpen . Even though they co-sleep at night the crib is still setup in the bedroom next to the bed.
For nap time I will nurse them in bed, then leave when they are asleep but of course I can see them in the video monitor. The problem I had with both boys is that they would get in the habit of falling asleep nursing and wouldn't fall asleep any other way and would wake up when I moved them.
With DS1 I tried everything I could think of including CIO trying to get him to take a nap in his crib. That is when we discovered using video monitoring was a life saver and I signed up for the Sleep Lady class too. I learned how I had basically taught DS1 how to fall asleep and only on me so what I needed to do was give him another "crutch" to use. I started giving him naps in the swing. It put him to sleep and not on me. Then we transitioned him over to naps in his crib. Then he had no issues and didn't have any issues either when he got booted from the bed when he was 1.
Probably way more info than you wanted but I think kiddos falling asleep BF'ing and co-sleeping start to get dependent on us for falling asleep and you have to find a balance of them learning to sleep on their own some too. Especially when they get older and would need to be napping in a crib or playpen. For DS2 I didn't worry about it until he was able to crawl then I just used the same process as I did with DS1 and getting him a new "crutch".