We are 15 heirs in an uncle's estate and we do not get along. Is it possible that an outside person manages the assets of this estate?

Yes. Either you agree to appoint one of you or a third party to manage the property of the deceased (contractual mandate) or in case of blocking of the succession linked to your disagreement, you can ask the judge of court of the place of opening of the estate, to appoint a judicial estate attorney.