If I'm completely in charge of running a contest (which I am not), I will make each and every contest to be as fair as possible for every participant!
How to implement that? Simple - I would preselect 1, 2 or 3 games from one provider for each contest (different month different provider), then run the contest on just the named games. No other games allowed!
A contest at any casino DO NOT allow you to play any game that you want does it? It tells you which game to play and only those games can be played - so every participant gets the same chance at winning, FAIRLY and SQUARELY!
If anyone doesn't like this kind of strict contest rules, no one is forcing anyone to participate!
If everyone prefers complete freedom of choice in the games they want to play, then there shouldn't be any game restrictions at all - play whichever game you want, as you want, anytime you want!
I would also BAN LAST DAY ENTRY of screenshots for any contest - the 1st screenshot submitted for any contest MUST NOT BE ON THE VERY LAST DAY!