Hmm, not sure if I’m understanding what you’re trying to do now.
That’s a battery powered smoke/CO detector. It has no interconnect function, but it can be set to announce the location of the alarm that is going off (from the speaker on the device itself).
https://www.amazon.com/First-Alert-SCO7CN-Battery-Operated-Combination/dp/B0002CWXZG
Do you mean SCO501CN-3ST? That’s also a battery-operated smoke/CO detector, although this one does have a wireless interconnect feature.
https://www.amazon.com/First-Alert-SCO501CN-3ST-Operated-Combination/dp/B000EVO7C2
So I’m confused by what you mean when you say your house is currently wired with SC07CN; that’s a battery-only device.
If you have several battery-operated detectors without interconnect (SC07CN), and you replace just one with a battery-only detector that has a wireless interconnect (SCO501CN-3ST), then what is it you’re hoping to accomplish?
Also just to be clear, the wireless interconnect on the SCO501CN-3ST does not mean it can connect to SmartThings or any other home automation hub.
The first alert zsmoke and zcombo work with SmartThings because they use z-wave, unlike the proprietary wireless protocol that connects a group of the SCO501CN-3ST devices.
Halo can be integrated with ST because it supports zigbee. I don’t have a nest protect, but I assume any integration with ST must be cloud-to-cloud.