We have just done this. the cot matress sit about a cm higher then our matress, but there is no gap. I have had to put a big long boday pillow down the railed side of the cot so that the matress is butt up against the bed. We also have the cot hard against the wall then opur bed...I know you cant do this but it stops the cot from moving!
Its great to ahve the extra room...cause DS1 likes to sleep wiht his legs in my face so having DS2 in his own safe space is perfect!

Plus OT but since Co sleeping form birth, i have never complained of lack of sleep cause i dont actually have to wake right up iykwim

Good luck hun x