Depends on what each is eating. If the betta will eat pellets from the bottom, add those first then some floating flakes for the others while he's busy maybe? Also, which rasboras? If small like chilis, they could be fed some frozen baby brine shrimp while the betta could be fed the larger frozen brine shrimp.
I haven't had a betta but know a few people that do. Many have no issues with their betta and shrimp. Just depends on the betta's personality, apparently.