Looking to install some new smoke detectors. I was going to install Ionization type smoke detectors in the lower floors and then a combination smoke and CO detector on the 3rd floor where the bedrooms are located.

I noticed that most of the combo detectors do not use ionization to detect smoke, but use photoelectric type sensors.
Is this a a big deal or should I stick with the most popular ionization type ?
Years ago I was told by a contractor to not install combo detectors, but to keep the CO and Smoke detectors separate ?
Does anyone have an opinion on that ?

If the CO detector has an interconnection wire for other alarms, should you connect it to the interconnection wire for the fire-Alarms ?
All this stuff is manufactured by Kidde, so I am assuming that is all compatible ?

Thanks
Sid.