How is the new instance intended to be used? Do the databases destined to be on it relate to those in the cluster at all? (please add this detail to the question so others will see it easily)
If it is not part of the cluster then isn't it just a stand-alone instance that needs no unusual care? You should just be able to install it as normal.
If it is being installed on the same machine as one of the clustered instances then be careful about memory use: instances are not terribly good at sharing memory between them so if the new instances is going on the passive of the pair it may claim much RAM and then if/when a fail-over occurs the clustered instance will find very little available when it needs to take over from the previously active one. There are settings to limit the amount of memory an instance can use, look into setting those if you are adding a new instance to one of the existing machines partaking in the cluster.