The 2nd Gen Vitz, (2005 onwards) isnt compatible with CNG due to the CVT transmission.
How ever Swifts shares the same engine as of a Liana (m13a) (a short gear shift). So i dont see why isnt it compatible with CNG when Liana is.
Yes there would be jerking misfiring in it due to the waste spark system, and dieing ignition coils.