Queries regarding upcoming trip

Hi everyone, I am visiting Iceland next week and have a few queries:

• I am planning to do the free walking tour in Reykjavik.
  • Would it be ok to give the guide a tip in USD or do they only accept ISK?
  • Secondly, what is a standard amount one would tip for these tours?
• Can someone who is in Iceland currently confirm if a heavy winter jacket would be required (example) or can one make do with a lighter down jacket with layering (example)? For context, while I am there I will be spending a few days in the city and also doing a few tours.