could one translate this 'for' as 'too' ('...too many months ago')?
No, 'for ... siden' translates to 'ago'. Your sentence would be "Jeg abonnerte for (alt)for mange måneder siden"
is 'for' always needed in that kind of sentence (talking about past)?
"X ago" is "for X siden" so you don't actually use "for" alone when you talk about the past.
If you wanted to say 'I have been subscribed for many months' would it be 'Jeg har vært abonnerte i mange måneder' ?