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.
Yeah, Kuhli loaches have a relatively small bio-load. My brother has two and they LOVE the NLS small fish formula pellets I feed them (though this may have something to do with the fact that he was feeding them Wardley flakes -_-). They even try to swim to the surface of the water because they're too inpatient to wait for them to sink :p
They seem to be pretty hardy considering his two have lived five years in his care (or lack thereof). He's been unsuccessful keeping anything else in that tank alive. However, they only come out when I feed them. This is probably due to the fact that he only has two.
I love watching them, though. I always find time to sit and watch his tank right after I drop some pellets in there.