I will assume that the OP is not using an agent, which is common for most on these forums. Gordon, this previous thread and accompanying links may help with the guide question:
https://community.ricksteves.com/travel-forum/tours/rome-tour-guides-agency
Whether individual guides will purchase tickets for you or request that you do that in advance may depend on the attraction and how difficult a specific ticket may be to get hands on (e.g. the underground of the Colosseum.) You'd have to have the conversation with a guide you're interested in about what you want to see? As well, even tour groups don't "go right in" to any attraction which has a security check, as no one can avoid those, but they do skip ticket queues. The same is true for independent visitors who have advance, timed-entry tickets.