Wow, I had the same question, but this thread helped a lot, seeing as how I'm a new betta owner too (Second night having him) and I'm slowly learning what all needs to get done and what can/can't go in there with him. Thanks!!
Kuhli loaches shouldn't be placed in a 5gallon since they do best kept in groups of 3 or more. I think you can keep an albino bristlenose pleco since they like slow moving warm water and are 3 inches max. They just need a place to hide.
nah albino bristlenoses usually stay under 3 inches, they rarely get above that. They are also extremely cute. The regular colored bristlenose, on the other hand, can get to 7in.
Clown plecos and bristlenose plecos are both small varieties.