Haf But will I be able to get a confirmed seat for my ‘Zurich to Chiasso’ leg. And will I be able to continue in the same seats from Chiasso to Milan
You do not need confirmed seats on that leg (as with allmost all Swiss trains). When you board in Zurich you will be at the start of the trains’ trip. Just get to the station a few minutes earlier, and just take your seats already in Zurich. Worst case you may have to move while en route.
You can book with SBB as indicated by Danhiel, but often booking only the Italian part with Trenitalia is cheaper.