A bit confused over compatibility
I currently have a Trek 1.5 which is set up with Shimano Tiagra so running 9 Speed, and on this I have the standard wheels it came with, which are coming to the end of life and also a set of fulcrum Racing 7. I have a friend who is looking to move on a 105 chain set plus a right hand shifter, left was broken in accident. As far as I am aware it will all be compatible. the issue is that there is also a Mavic askum wheel with a 10 speed cassette available as well, firstly could I run a 9 speed cassette on the mavic wheel and continue with the tiagra shifters or could I run a 10 speed cassette on the fulcrums and run the 105 shifters, which would mean looking for a 105 left shifter and also, Would it be worth paying out for the left 105 shifter and 10 speed chain.