I'll be in Rome early October. Is it necessary to buy tickets for the Colosseum and Forum ahead of time-like from home?
Yes, it's easy to buy tickets from home. Keep in mind that the window for your dates opens about 8 weeks in advance of when you would like the tickets. All tour experts advise purchasing in advance. Even with the Roma Pass you need to reserve a time for the Colosseum and pay a booking fee. I checked out the Roma Pass. It didn't make sense for our trip. Do the math (how many museums, how many metro/bus rides) to decide if it is right for you.
As Evelyn said, I'd absolutely buy tickets in advance and, yes, even with a Roma Pass you're now required to make an advance, timed-entry reservation. This is the #1 tourist attraction in Rome so the ticket line - if you haven't already purchased them - can be very, very long. Even the security-check line, which can't be skipped, can be lengthy. Coopculture Tours which include parts of the structure which are otherwise inaccessible to the independent visitor, such as the underground, can sell out within a few hours of becoming available.
I highly recommend purchasing advance tickets or tours of the Vatican Museums as well.
Thanks everyone! Your replies are appreciated. Now if they can just get this corona virus under control!