What do you mean by ‘booked’? Unless you have a train specific Supersaver ticket then you can take any train with a standard ticket so you do not have to look at the clock.
I bought a ticket online from Trainline a Regional Express ticket with a time slot of 8.00pm
When you land in Zürich you will go through Immigration as you are entering Schengen at Zürich. When you land in Geneva you have to get your bags and then go through Customs - which normally means just walking through the channel and out of the Customs Hall. Then you have to walk to the station which takes about 3 mins and find the platform for Montreux and go down there.
Normally speaking at GVA airport I can safely be on the platform waiting for me train with my baggage 30-40mins after I land. What time is your flight scheduled to land?
In future don't ever buy "timed" tickets when flying in... if you miss the train because your plane is late you have to buy a new ticket. For Montreux that means paying about 30frs so it isn't the end of the world!
Thank you so much for your response. I will be arriving at 6.50pm. I thought it best to get tickets first as it was an evening arrival. I really should have checked this forum first. Will definitely use TripAdvisor for future travels.
Regarding your ticket:
"I bought a ticket online from Trainline a Regional Express ticket with a time slot of 8.00pm"
No, that is probably not what you bought. When you buy a ticket for a Swiss train on Trainline they just give you a normal. I believe they use the DB system to issue it. Have a look at your ticket.
It is true that buying a train ticket starts with looking up a schedule, but that does not mean that the ticket is limited to the train you picked at that time. Your ticket is thus going to be valid all day, on any train from Geneva to Montreux. So just take the first one that you can reach after your arrival. Use www.sbb.ch to look up the schedules. You will see that you have plenty of trains to choose from.
Swiss trains are mass transit. Taking the train from Geneva to Montreux is like taking a train from Perth to Freemantle. A trivial trip that thousands to every day, without advance planning.