When upgrading Xcode, it wil overwrite the old one including the iOS simulators. To install an old version of Xcode:

1. Download the .dmg from the Apple devcenter.
2. Open this package so you see the Xcode.app.
3. Create a directory for this Xcode version, e.g. /Applications/Xcode6.4 or some other name.
4. Put the Xcode.app into this new directory.

To show the current default version of Xcode:

xcode-select -p

To set another Xcode version as the default one:

sudo xcode-select -s /Applications/Xcode6.4/Xcode.app/Contents/Developer

Simulators

You can download simulators from Xcode / Preferences / Downloads. When you still don't see devices after installing, add them from Xcode / Window / Devices and click on the little plus sign.