I am dealing with a large telecom which purchased the small telecom I signed a contract with.

Despite signing only 1 contract with them, the two racks, and the bandwidth which feeds 1 rack (I connect privately to the second rack at no charge) all have different termination dates.

Check your billing. Did your first invoice have all of your services? How

about the second one? If they didn't bill you on time, it's partially your fault. If they did bill you for everything, then you have empirical evidence showing when you started the term (by performance.. paying) and their dates are wrong.

How signing one contract resulted in 3 different end of term dates, months apart, I can't quite figure out.

Did your original vendor provide a turn-up or activation notice? This should indicate the date that would go in their system to indicate term expiration.

Can anyone point me to a mailing list or discussion forum containing advice on dealing with such issues? And the wider issue of negotiating good rates with telecoms?

Typical economics, more volume leads to better deals; growth appeal; competitive reference, etc.