>> Which website do you recommend getting reservations for the Gothard Express and Golden Pass? (cheapest price that is) Reservations on Gothard are mandatory?
There is never a discount on seat reservations unfortunately!
Gotthard Express reservations (mandatory):
www.myswissalps.com/go tthardpanoramaexpress/ packages
Golden Pass reservations:
tickets.goldenpass.ch/ pls/apex/f?p=151:1:67048311426951 56::NO:::
>> I will be travelling from Interlaken to Montreux (with Swiss Pass 2nd class) - Do I need to make reservations from Interlaken Ost to Zweisimmen?
Reservations are not compulsory, but if you want to be sure of sitting together and getting a window seat, reservations are a good idea.
>> I assume there will be plenty of seats?
Not necessarily. Did you read this recent trip report?
>> Is it first come first serve when you get on the 2nd class trains?
Yes, if you don't have a seat reservation.
>> if I want to sit 1st class from Zweisimmen to Montreux? I heard it's worth it for the view?
The VIP seats at the very front of the first carriage, where you get the view from the driver's perspective, are worth it for the view. Out of all the first-class seats, only 8 of them are these VIP seats, and only 4 of them are in the front row (see attached diagram, with VIP seats circled in red).
Not all Golden Pass trains have the carriage with VIP seats. From Zewisimmen to Montreux, only the 12:25 and 18:25 departures have carriages with VIP seats. If you leave it too late to reserve, all the VIP seats will be taken.
>> How do I upgrade and reserve the front seats?
You can buy a class upgrade from any staffed railway station in Switzerland.
Reservations are best done online if you want the VIP seats. You can do this before you get your class upgrade, but must have both the reservation and the upgrade when you board the train.