My advice would be to not worry about it too much if your currently happy with the situation. Co sleeping with DC of different ages can work just fine.
I do bedtime on my own with 5 year old twins and 10 month old baby. I used to do stories and bed in the twins room and then let them come in to us when they woke, but once the baby was borne found that too hard.
Now, the twins and the baby and I read stories in my bed, and the three children all fall asleep there. When my DH comes to be, he lifts the twins into their own beds, and we cosleep with the baby. One or both of the twins invariably joins us at some point in the night.
What I did ensure is that the baby had a safe place that was OUT OF BOUNDS to the older ones. In our case it is a cot with the side down, next to my bed. When baby was little, he would be in the cot, and I didn't let anyone sleep between me and the baby. That's a bit more fluid now the baby is bigger, but I am still careful about who sleeps where in the bed and what covers/pillows they have.
In your set up, I think I'd start to transition your older child out of the cot. The baby is likely to need it within a few weeks/months of the birth, and I would worry the older child would feel pushed out if they suddenly had to give up the cot. Could the older child fit in the main bed between you and DH? Or is there space for a single bed alongside your DHs side of the bed? Then you could put the cot away for a bit, and get it out again later when the baby needs it. It would hopefully help your oldest to not feel pushed out.
In our experience, the baby crying very rarely wakes the older two, even if they're in the bed with us. I guess we're all used to movement/noise in the night, and so sleep through things that we don't need to wake for?