Can I pack everything I need for a 1 week trip to Rome on one of Rick's city tours in the day bag or am I being overly optimistic?
I think the is very optomistic, unless you plan to buy some clothes (and a small duffle bag to bring them home in) over there. This could be fun. But Italian women are chic, and Rick's city tours pack in a lot of activity and there isn't much time for shopping. so unless you plan to arrive a day or 2 early and pick up some things, I think a small wheeled carry on suitcase is the way to go.
Though if you just brought a few things in the daybag, I guess you could wash them frequently.
Even though I do laundry regularly in my hotel room -- it's not an activity I want to do virtually every night, and I think you would be if you bring only a couple of things. You will be walking a lot, clothes get sweaty, dusty and dirty. You will want to have fresh clothes often, I think.
I would suggest Rick's 21" Roll-Aboard use the Civita for day use.
If you live in Seattle, you might want to go up to RS offices in Edmonds to check things out including a class on "packing light" that is given regularly,I took it twice!
You should expect to be told that a day bag is not sufficient for your trip and you should buy one of his 21/22" roller bags which I did and I get along fine for a month there each summer.