the information on it that I've found so far doesn't have any information on this aspect. it appears they had desvretion to decide which setances to pass up to life imprisonment but did it have broad powers of pardon ?
Yes. Article 27 of the Statute of the International Criminal Tribunal for Rwanda, made under UN Security Council Resolution 955 (1994) states the following:
Pardon or commutation of sentences
If, pursuant to the applicable law of the State in which the convicted person is imprisoned, he or she is eligible for pardon or commutation of sentence, the State concerned shall notify the International Tribunal for Rwanda accordingly. There shall only be pardon or commutation of sentence if the President of the International Tribunal for Rwanda, in consultation with the judges, so decides on the basis of the interests of justice and the general principles of law.