[Exchange rings in Kyoto station]

As long as you are not blocking traffic, I don't see why not. Please be considerate of locals going about their business. The station is large so you will have no problem finding a place out of the way of the crowds.

[Staying in Yokohama when visiting Tokyo]

It really depends on where you want to visit. Planning to hang out as much in Yokohama or places south of Tokyo as in Tokyo itself? 

I'd split one week in Ueno (Ueno Park, Zoo, museums) or Asakusa (convenient for Tokyo Skytree, daytrip to Nikko) and one week in Yokohama with daytrips to places you mentioned.

[Sapporo to Otaru pay by Suica card?]

If I take a train from Sapporo to Otaru do I need to buy a ticket or can I use my Suica card I got in Tokyo.

Thank you.

Elizabeth