Book an open jaw flight ticket - In Swiss out Rome, perhaps (or the other way round)
If you only travel less than 3 train trips, perhaps you should consider buy tix there, no need any rail pass (my own experience)
You can do own math - swiss train online can search for the price; then compare the rail pass you may purchase. Rail pass need check with SG local agent.
How do you intend to travel? touch and go or you want to really experience the cities?
- travel outside the summer season like may and october, and you will enjoy lesser crowd and cheaper accommodation
- get a eurail pass and you have the flexibility of traveling anywhere you want, it's not cheap but worth it if you utilise it for long distance journey.
- it is not dangerous if you are street smart and try not flash your expensive camera and phone especially in the train stations and touristy area. A very good advice is too act blur if someone for no reason bump to you and ask whether you speak english, from my experience 7 out of 10 times they are trying to sell you something or scam you, especially in popular places like rome, paris and barcelona.
-buy a good guide book like Rick Steves or lonely planet to research on what is your interest. Different people have different interest, doesn't mean I like to visit museum you will be interested in visiting the Lourve? So write down a list of the must see places and you will have more a less an itinerary!
IMHO, if you have 16-18 days in europe and is fit and ready to cheong. My itinerary will look something like this:
Fly in paris (3N)
-champs de elysee
Train to lucern (3N)
-cruise the lake
-daytrip to jungfrau/ interlaken
Train to venice (2N)
-st marco square
Train to florence (3N)
-duomo, piazza michaelanegelo
-daytrip to siena
Train to Rome (3N)
-Fly back from rome
This will give you a nice brief overview of europe, you can minus or add some days in the main cities