The tour is excellent. If you can, book through the official coopculture.it site. Third party vendors buy the tickets there, charge you three times the amount, and just deliver you to the official tour. The problem is that third party vendors use bots to scoop up the tickets the instant they are available on the website. However, coopculture releases additional tickets closer to the date. So, keep checking.
By the way, there may be more tickets in Italian and Spanish. When you are in the Colosseum you could stand with one of the English groups instead.