A school of six neon tetras would be perfect in a ten gal with a betta. Provided they have lots of hiding places.
Even though I would recomend this, I believe I should warn you this arrangement doesn't always work out. Some bettas are ment for solitary life others do okay with friends, it really depends on your bettas personality specificaly.
Even though there's a 95% chance this arrangement should work but make sure you monitor your tank closely for tailbiting or harassment.
Best of luck!!!
Rest in Peace: <3
Mack <3 (peppered)