We are just started trialing
hugelkultur beds with brazillian pepper. Brazillian pepper is allelopathic, but I am not sure how bad it will affect the other crops. Using them as a border may have no strong consequences, but there is only one way to try... Trial! However, DO NOT use the stumps, as they will re-sprout.
If our trial hugel beds work, we will be doing a 2 acre area of
hugelkultur beds to eliminate plastics for
irrigation.
Good luck