IMO/E-using deshelled pea pieces on occasion for constipation issues won't cause any problems for a Betta, however, you don't want to use peas as part of their staple diet due to poor nutrition. Since a Betta has a short GI tract the pea passes too fast and can't be absorbed nutritionally.
In my many years of research on this species as well as other species of fish-Very few are true carnivores. More are omnivores and eat both veggie and meat. In the wild, Bettas are seen eating some species of algae. When stomach contents are examined in wild fish-this is the best method to understand what their diet really is. This is the information I look for when I have a wild critter baby that needs to be hand reared-stomach contents of wild adults.