5 UK gallons is ~6 US gallons. Basically the same and still way way too small. You're going to need like 5 times that much, plus mega filtration.
I have 3 ADFs in a 5.5g tank, unfiltered. They're fine. The general rule of thumb for ADFs tends to vary from place to place, from 1g per frog to 2g per frog. I like the 2g per frog method myself.
For what it's worth, I never see the frogs interact with each other, unless it's feeding time, then all they do is fight over worms. I've also spoken to other people who have very active frogs, living on their own, in community tanks. The most crucial factor in keeping a happy frog is making sure they're fed.
They're massively cute, and stupid... and blind.
In my experience, they're very active in larger tanks, and the smaller the tanks get, the more sedintary they are. When I had the same 3 frogs in a 10g tank, they were all over the place. After I was forced to move them to a 5.5g, they stopped being as active. They're still extremely active and in my opionion, as happy as a frog can be.
You can probably get away with a single ADF in a 3g, but after that you'll be at your biological max. You'll need to be right on top of water changes, doing at least one 100% change per week, two would be ideal. You'll also need to hand feed the ADF, as bettas will outcompete them for food without even putting any effort into it.
Your betta will enjoy the frozen bloodworms you'll feed your ADF, and their shared diet is part of the niceness of the ADF/betta community.
As far as betta aggression, it varies from betta to betta. All my boys are fine with ADFs, but my girl Uta did try and kill them after 3 months.
