My roadbike has 9-speed Tiagra gearing at the moment with a compact up front and an 11:25 on the back. Generally that's fine but I'm thinking about trying to do my first 100 miler soon, and would prefer considerably lower gearing for some of the climbs later in the route when I'm knackered.
Can I just slap one of my 11:32 or 11:34 cassettes and a 9-speed MTB mech on there temporarily? Will that work ok with the Tiagra STI?

