Mobile applications can only be installed manually avoiding the different controls applied by Google play and the Apple Store. Will there be guarantees that the applications comply with the different security standards?

Welcome to the forum tipejo

One should always be wary when installing any application to one's phone from outside sources. There are no guarantees even when installing an app from the 'official sources', if you have seen the news about apps containing malware in Play Store.
Poker apps can be good or bad but I don't think that you can go very much wrong if you are using an app that comes from e.g. Pokerstars. Still, having a good antivirus on your phone is a must in my opinion.