Is it allowed / google compatible to show an interstitial or video ad just after starting the game app ?
It is not against Google Play policy.
Can somebody else confirm this please ? It is NOT against google play policy ? Is this right ?
Confirmed. It’s NOT against Google Play policy.
For Admob, you need to display loading screen before showing your ads (loading - show ads - main menu). For other ad networks, it was ok to do so, and it is not against google play policy.
Hi tobiblueeyes,
Yes, you are allowed to show an interstitial/video ad just after the start of an app 
Keeping user experience in mind, I would suggest placing ads strategically throughout your app rather than abruptly when the app is first opened. For example, showing them during a natural break in the application’s content, such as after completion of a level in a game or between page views, would be less intrusive and the user would be more likely to interact with the ad (helping you to generate revenue).
Hope this helps! Best of luck