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.