Answer

Saying selichot is a custom, and the practice is to say them with a congregation. But that is no worse than prayer, which can also be said individually even though the practice is to say it with a congregation.
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Questioner:
If the practice is to say them with a congregation, doesn’t the custom itself become binding? (“Do not forsake,” “the custom of Israel has the force of law,” etc.) And if it is binding, what did you mean when you said they can be said individually?
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Rabbi:
Indeed, the practice is to say them with a congregation. But the same is true of prayers as well, and so by simple reasoning it seems that selichot, which are a custom, are no more stringent than prayers, which are an actual obligation. From this it follows that even if the usual practice is to say them with a congregation, the main custom is the recitation itself, and the congregation is not indispensable. For the same reason, it is also permitted to say them outside the synagogue, even though the usual practice is to say them in the synagogue. That is not an essential element of the custom.
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Questioner:
Is there any problem with saying the entire selichot text when alone (falling on one’s face, the Thirteen Attributes, passages in Aramaic, etc.)? Or do certain sections need to be omitted?
 
You wrote above that prayer too may, from the outset, be said individually. How does that fit with what is written by Maimonides: “Congregational prayer is always heard; and even if there are sinners among them, the Holy One, blessed be He, does not reject the prayer of the many. Therefore a person should associate himself with the congregation, and not pray alone whenever he is able to pray with the congregation. A person should always rise early and go in the evening to the synagogue, for a person’s prayer is not heard at all times except in the synagogue. And whoever has a synagogue in his city and does not enter it to pray is called a bad neighbor.” It seems to me that the other halakhic decisors also follow this line, that prayer with a congregation is obligatory.
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Rabbi:
The Thirteen Attributes are said only with a congregation.
 
On the contrary, Maimonides’ wording indicates that this is not an obligation but an advantage. In the Shulchan Arukh as well it does not appear as an obligation (although some have inferred that from it, and I am not sure there is much substance to that).