Female necks are significantly shorter. Speaking from experience, most costume heads choke me because I have a shorter neck. Provide an emergency kit for the performer which includes safety pins, thread and needles, scissors and cloth tape in case something gets ripped or torn while at an event. Everyone has their own personality, and that includes those to step into costumes and deliver lines non-verbally. A sense of timing, a sense of fun, a sense of safety, and a sense to dance are great things for a performer to have.
Today there are many liability concerns as well.
Allow this person to develop a strong personality for the character. When your sponsor calls up to book a mascot appearance they should be getting the best entertainment package possible so that your actor can make money for their time, and your organization ca benefit from the performer's growing reputation as well. Your mascot should be aware that any inappropriate behavior on their part will be their responsibility to handle the consequences. Picking fights with fans or other mascots, roughhousing, lewd gestures, etc. should be talked about before they ever represent your organization. Be patient and find out both perspectives when the mascot is involved in an incident.