I wouldn't use the co-sleeper this way. As secure as it seems, it is just not worth the risk.
One thing you might want to consider is, my middle boy had the hardest time transitioning and I found out that it was because I moved his room. Once I moved the crib instead of him, all was good. He felt like his little sister was taking his stuff when I tried to move him. In short, move the big boy bed into his room and the crib into the big boy bed room. Also, another thing I did, was get netted rails for my son's big boy bed, it gave him the crib feel and kept him from rolling out.
Good luck.