According to the Timetable tab of the page you mention, there is no service from Rome to Florence included in this pass. However, this does not mean that there is no bus service at all between Rome and Florence. Eurolines Italy has regular services between these two cities.
The countries you mention have excellent train services. As a general rule, trains are more expensive than buses. They are usually more comfortable and faster. Note that these are trends and no universal laws. Trains or bus? It depends on you and the importance you give to criteria like price, comfort and speed.
Regarding bus services, the Eurolines group has the most complete network in Europe. However, there are other more localised/specialised companies. How to find out about them? Do searches on the internet, have a look at travel guides, or ask precise questions on sites like this one (e.g. Which intercity bus companies are operating from Florence? Which companies are operating buses between Rome and Paris? etc.) . I think that outide the Eurolines network, there is no centralised resource on bus travel in Europe, such as there is seat61.com fro train travel.
Also check if a pass is worth the money. You seem to have an idea of where you want to go. Check how much it would cost to buy individual tickets rather than a pass. Feel free to combine bus an train in this case. If you are planning and booking ahead, you can make some good deals. Passes are not necessarily the cheapest option. You have to eat lots of kilometers to make them profitable. However, they give you more freedom. That can also be worth some money.
There are two extreme approaches to planning a trip. (1) Take a pass, look where it lets you go and then plan your trip accordingly. Or buy a pass, plan nothing and let yourself drift along the routes included in the pass. (2) Make a list of places you do want to see and the plan your trip accordingly. Buy individual tickets for trains or buses, whatever suits you best. Possibly combine them with more local passes.
Don't hesitate to dig this site. It has a couple of questions that might be relevant to you, such as e.g. this one.