I'd have to agree with Crispii, JuiceBox and Byron.
Betta's are quite chill when it comes to swimming around the tank, the congo's would dart around and eat the vast majority of food before the betta could even get a sniff.
Also alot of people (including myself) recommend not having other species of fish with a Betta as they can be aggresive, they'll harrass attack and even kill other fish. You'll find alot of places on youtube and the internet that say they are compatible with "x" species, however, it's still not recommended.
Every fish is different. I saw a youtube video where a betta lived happily in a community tank, there where ghost shrimp and some tetras if I remember correctly, they were all happy, no chasing or fin nipping at all. The owner introduced cherry shrimp into the equation and the betta destroyed them all, he didn't even eat some either just killed and left them, then started turning onto the other species that had been in there since day one. What I'm getting at is this, your betta very well may be okay with others in it's space, but would you honestly take the chance to introduce other fish into it's territory?
I know it's not nice to hear, but we'll help you as best as we can to make sure your fish are as happy and content as they can be. you could always buy a smaller tank to house the betta on it's own, and start a community fish tank in your 50g.
Either way I hope all gets sorted and you can get the fish tank you visioned
Good luck
Edit - added part to the betta story, and some typo's/clean-up
