Vatican Museums: the benefit of a tour here is being guided to the highlights and having direct access from the museums/Sistine into the basilica via the passageway reserved for tours.
I might suggest pre-booking the 3-hour combo tour of museums and basilica offered on the Vatican's website (you want the €37.00 selection for individuals):
http://www.museivaticani.va/content/museivaticani/en/visita-i-musei/scegli-la-visita/musei-e-collezioni/musei-vaticani-e-basilica-di-san-pietro/visita-guidata-musei-e-basilica-s--pietro-per-singoli-e-gruppi-.html
If you happen to be in Rome on a Friday, then I'd suggest the 2-hr. night tour (doesn't include the basilica):
http://www.museivaticani.va/content/museivaticani/en/visita-i-musei/scegli-la-visita/musei-e-collezioni/musei-vaticani-e-cappella-sistina/-i-aperture-notturne--i---visita-guidata-musei-e-cappella-sistin.html
I don't see the need for a guide at the Colosseum: just rent audio guides there or download Rick's free guide:
https://www.ricksteves.com/watch-read-listen/audio/audio-tours/italy
DO pre-purchase ticket for the Colosseum: ticket lines are very long! Your ticket also cover the Forum and Palatine, both of which are well worth seeing: € 12.00 + €2.00 reservation fee.
http://www.coopculture.it/en/colosseo-e-shop.cfm
Tours of the Colosseum only are also available through Coopculture for an additional € 5.00 but must be reserved by phone:
In English:
from Monday to Sunday
10.15, 10.45,11.15, 12.30, 13.45, 15.00, 16.15, 17.15
purchasable by phone:
+39 06 399 67 700
If wishing a guided tour of Colosseum, Forum and Palatine, this one comes highly recommended by many RS posters:
https://www.walksofitaly.com/rome-tours/roman-colosseum-tours
This one is highly recommended for the Vatican as it gets you inside a little before the mob (and it WILL be mobbed and hot):
https://www.walksofitaly.com/vatican-tours/pristine-sistine-chapel-tour
As far a general walk, Rick's "Heart of Rome" audio walking tour (same link as above) could serve you all well: no physical guide needed!
Florence: I don't see a big need for a guide anywhere here. With a good guidebook and audioguides from some of the bigger attractions (like the Uffizi) you should be fine. Pre-ordering tickets for the Uffizi and Accademia is a must during high season, and Rick offers a selection of free audio tours for Florence as well (same link as above).
http://www.b-ticket.com/b-ticket/uffizi/default.aspx
If you are leaving next month, you're a little late in the game for snagging tour-group and/or entry slots for 7 during high season so I'd get on it!