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
i agree with everything said cept it was DD for me. Oh and we rolled up a blanket to jam against the rail side and make sure the matresses had no gaps between them. Our matresses didnt match up in height either - I thinkDD's was a few cms lower than ours but it didnt cause any problems....

We coslept from birth and she was moved into the side car cot at about 18 months i think it was - it was summer, she is sweaty and the feet in DH's face and head on mine thing was aggrivating us cos she was too long at that age and DH was always half off the bed!

We had a couple days dramas as she adjusted to not being in the middle but after a couple of weeks she was willingly rolling over into her cot in a half sleep state after coming off the boob, so then she started appreciating her own space.

HTH!