if you really buyed a ticket, you are fine and dont need the interrail pass at all.. but I guess you buyed the compulsory reservation, which means you either need to change it to second class, or perhaps you can buy an upgrade to regular conditions (which will be pretty sure pretty expensive)

little info, you only need reservation from Domodossola/Chiasso to Milano. so you could also use the EC to one of this border stations and then take the regional train that doesnt need reservation.

Yes I bought the reseravtion tucket on Oebb... I will see if I can cancel them

Thank you for confirming! 

You can use the allocated seat only by paying extra the difference (on full fare) 2nd-1st. Swiss trains have sky-high prices, so I think you will not wish that.

Mostly the direct trains here cost 11€ extra (incl the REServ), but I am unsure if this is also for 1st.

There are 2 options and you do not have to worry now much about the oso much feared fullness:

1.when IN the swiss go to a counter (SBB), explain it and ask what they can re-arrange. But that may not be possible. In the right area they will even speak francais.

2.the extra cost is ONLY for direct EC trains Swiss->IT: use any Swiss train till border (for most thats Chiasso, for free, get off, wait for next local train to Milano=also free. You forfeit what you paid, but there are no extra charges either.

In this case and if you have the time, you could even go via the old slower route on/via old Gotthard line (new line is a very long tunnel), via Airolo and thus see some more of the wonderful views the Swiss has so many of.

Thanks so much for your help!

Another quick question, how come the itinerary to italy you guys speak of is not proposed on interrail? My final destination sis firenze and the only way for interrail is to go from basel to milano and milano to firenze

And i would need to pay for local train ie milano to firenze train

Thanks again! 

@Linda68 

The Interrail/Eurail planner is only updated once a month. The advice from the experienced travellers in the community is not to use the planner for planning your trip, but to use the timetable from the national railway companies. Some of the bigger ones, like DB (the German Railway), SBB (the Swiss Railway) and ÖBB (the Austrian Railway) cover several countries. 

The advice from the experienced travellers in the community is to use other ways to make reservations than the Interrail/Eurail website. You can look at the guide in the link:

https://community.eurail.com/train-connections-reservations-47/how-to-get-reservations-105

If you, after having looked at the guide, have questions about how to make specific reservation, please create a new topic with your travel details (departure date, time and route) and you will get advice.

The journey planners by default give the quickest most direct connections. If you want to get itineraries that use slower trains to avoid reservations or see scenic lines that are bypassed by high speed trains you have to use the more advanced features on some of the journey planners to exclude High speed, Eurocity, IC trains and sometimes also add in an intermediate point that is on a line you wish to travel through.

The German rail planner is one of the best for most of Europe for planning (but not always for booking Interrail reservations).

https://reiseauskunft.bahn.de/bin/query.exe/en?

Of course choosing local trains and scenic routes, especially on a long trip will make the overall journey much longer so you need to decide which you want to prioritize, the fastest journey or a scenic trip.