So 10 gallons. Going off of 1 gal for inch of fish.
3 Loaches= 12 inches
1 Betta= 2
Needing 4 gallons won't be a problem in a planted tank but getting even 1 more loach would bring it to you needing a 20 gal and if you wanted 5, you need a 25 gal but could stay at about 15-20 with the plants. That's just my opinion. Not to mention the loaches will eat the snails and then need food, they eat a lot more then a betta and different food. They can eat betta food but as bottom feeder wafers and pellets contain a lot of algae (often first or second ingredient) it would be to much plant matter for a betta to deal with. My opinion, I wouldn't do it.