Can a tourist buy a Swiss rail pass on arrival or has to purchase it online before hand? Which of the above two options are better given that I arrive at 7 PM on a weekday?

Here's the exact page with your options: http://www.sbb.ch/en/leisure-holidays/holidays--short-breaks-in-switzerland/swisstravelsystem.html. Railway ticket office opening hours: Zürich airport (until 22:30), Geneva airport (until 20:15) and Zürich main station (until 22:15)

'If you have already planned your travel programme in detail, it is advisable to purchase the Swiss Pass in advance. There are points-of-sale in many countries and you can also order the Swiss Pass online. Prices may vary depending on shipping fees and currency exchange rates. Please bear in mind that the Swiss Transfer Ticket is not purchasable in Switzerland. If flexibility is important to you and you wish to choose when and where to travel once you are here, we recommend that you buy the Swiss Pass in Switzerland. They can be purchased at all main railway stations.' http://www.swisstravelsystem.com/en/support/Frequently-Asked-Questions.html