SMN/BLM is reletively weak and useless because Blood Pacts(your Avatar special moves) is a 1 minute downtime before you can use again, a WHM sub will help you during that downtime. A BLM sub cannot hurt anything because its power is half your level.

Each job and race has a base, if you switch jobs say to a WHM but you already lvled it to 20, you will go to lvl 20 WHM even if you have say a lvl 60 SMN.

You can only switch at a nation's Mog House or in a town's Nomad Moogle.