I'm developing a chess game and for the Elo system I want to know recommendations for the starting Elo level for the users.
Your starting Elo will influence how well your Elo compares to Fide Elo, USCF Elo, or the rating of other sites. The internal functioning as rating isn't affected at all.
If you want to stay close to Fide Elo you could perhaps ask the user to rank himself (and then subtract 100 points … ;-)). Otherwise you'll have to guess what the average Fide Elo of a user might be. So that depends on whether you cater to beginners or to experienced club players and is probably the reason why starting Elo varies between 1200 and 1800 on different sites.