Pellon #71 Ultra Firm Interfacing
 

Is this too stiff to use for the handles and the bottom of a purse/bag?

Depends... It's not too stiff for a bottom insert. The handles depend on how they are constructed. If it's just a single layer of interfacing, should be OK. If the design of the handle results in multiple layers, it all depends on if your machine can handle the bulk.

Would you trim the interfacing about 1/2" smaller for the seams not to be bulky or
would it be ok.