Star Trek

[Alex]

(isn't that one of the basic rules of alternate realities...that you're never allowed to meet yourself without suffering some dire conequence?)

I don't think there are "basic rules of alternate realities" though you're right that such a rule is not infrequent in the genre. Don't think it has ever been suggested for Star Trek though (and in reality, to the extent that alternate realities are hypothesized to exist, transfer of information from one to the other is also hypothesized as impossible so the issue can't arise).

Besides, old Spock is no longer the future of the current Spock, he's just another person.