Could I use my iMac 2010 (mini-DisplayPort) and Windows 10 as a second monitor for my Mac laptops?
No, it cannot be done while the iMac is running Windows 10 natively.

The feature you're looking for is called "Target Display Mode". It only works when the iMac is booted into macOS.

You can read all about how the feature works and its requirements here:

https://support.apple.com/el-gr/HT204592#requirements