Good point and in fact in a monastery it may be far more important than whether it has fewer or more members who are orthodox or not. I can only speak from the Benedictine point of view; I imagine it’s similar for Cistercians though (same rule).
Monks vow stability, obedience, and conversion. That’s all. They don’t make a promise of “orthodoxy” although as an order Benedictines profess obedience to the Holy Father; that’s embedded in the promise of obedience. They don’t make a profession of always having the correct opinion on theological matters. They make a profession of obedience. That means what the abbot says, becomes final. Obviously though an orthodox abbot is essential.
In the abbey I’m associated with, there are more orthodox and less orthodox monks. The abbot is very orthodox though, and the monastery follows good practices for a contemplative abbey: fidelity to the Divine Office including chanting the entire psalter in a week, Gregorian chant, correct liturgy for the Office and the Mass, etc.
And more important, the community, including the less orthodox monks, are obedient. They made that vow and they take it seriously. Everyone recognizes that the conversion part will occur at different speed for different individuals. So there will be monks in the community at different levels of understanding. They are there to study the rule, seek God, and climb the ladder by lowering one’s self into humility.
To me, seeking a community one should first look at the abbot. It is to him you will be giving your obedience. Then at the community itself. You will probably find that far more annoying than the monk with the weird theological opinions might be the monk sitting next to you in the refectory for 60 years who slurps his soup. That far more than anything will be the test of your command to love your brothers like your family. So equally important to the abbot will be the degree of fit with the community, and keep in mind the community as a whole gets to vote on whether the postulant becomes a novice, and whether the novice becomes first a simple professed then a solemn professed. They have to accept you as much as the other way around.
Seeing young postulants come and go, my view has been that postulants that show up and want to join our abbey because it’s traditional (it is, in the classic Benedictine sense, not in the sense of which liturgy it uses, which is OF; but it retains faithfulness to the magisterium and classic Benedictine traditions like Gregorian chant), fail and leave. That’s not what monastic life is all about, and any good vocations program will shake those out early in the process.
A monastic vocation is to seek God in the Benedictine way through the promises of stability, obedience and conversion. What matters is finding a community that is true to that ideal, not one with 100% orthodox members. All my opinion of course… and orthopraxy as Cristiano points out will be far more important: orthopraxy in how the community carries out its daily life, through faithfulness to the Rule and its obligations particularly obligation to the daily liturgy. If that’s in place, the community’s members will experience spiritual growth, even if some (even many) are not “orthodox” in opinion. It boils down to it being “not all about me”. One of the least orthodox monks I know is also one of the most faithful in being obedient to the daily requirements of monastic life and to his abbot. That more than orthodox thought will IMHO serve as an example to novices.