It looks like the OP has already bought his alarms, but I'll add my bit anyway. When we did our major renovation we needed 10 interconnected hardwired smoke alarms. The CO alarm could be separate, at least at that time. I bought the most popular ones at the Big Box store that came in the multiple builder packs. After the second middle-of-the-night false alarm in two years, I did some research to prevent false alarms. I bought a set of Gentex 9132 photoelectric smoke alarms (since discontinued and replaced by the S1209). They were expensive, but I've certainly slept better in the years since then. Unfortunately, Gentex doesn't market to consumers, so they can be hard to find.