I take it from your post you have already invested in the Sony Headunit, oh well.
you are right in that you want to start with yout front doors, then if necessary move to the rears.
two ways are full range speakers just as components are but in a single piece rather than separate tweeter plus crossover. The coax's use a capacitor to filter low-frequency notes from the tweeter - but let the mid run full range - not good. the crossover in the components cuts the high frequency from the mid and the low frequency from the tweeter. I cannot reccomend components over coax's enough.
The non bose 156's actually have a VERY cheap component set with a tweeter in the door handle (using a capacitor to protect the tweeter) and the mid in the bottom corner. The bose systems have the tweeter integrated into the mid.
You will not want to put a set of coax's in the front without disabling the tweeter otherwise you will have way too much treble.
The front components are key, sat in the front seats over 70% of the sound you hear will come from them, with the rears adding a bit of depth.
There is better kit out there than the pioneers, they are not brilliant speakers.
In my old 156 I had a set of infinity 655cs components and these sounded brilliant in the car, the new model is linked here
without having to hack at the door pod I could fit the inifinity's or kenwood dual mags (which I didn't liek the sound of), ALpine's wouldn't fit nor would focal's. did not try the pioneers (its the depth plus diameter of the magnet that count).
If you do front components you can leave the factory rears for a while, or put some cheap coax's in...
In the old 156 I took the door cards off
start by removing door handle cover, take out little cap, remove crosshead behind it. hold handle open and slide cover back and out.
remove speaker grille, remove speaker (4 screws)
There are two screws holding speaker pod to metal of door, remove these.
in the bottom of hand hold in the door where the window buttons are there is a little rubber mat, take this out there are 2 bolts, (torx T40) lift the unit with the window buttons out, disconnect (make sure you knwo how it goes back together) and put to one side. There are two bolts holding the frame to the door undo these.
The door is now held with sets of clips at side and bottom, which you can lift by gently prying door card away from door at bottom/side. when these are released, the door card can be lifted off the lip at the top of the door.
Using adhesive velcro you can attach the crossover to the doorcard. wire mid bass signal to crossover in run 2 pairs of wires one to the mid bass mount and the other up to the door handle hole.
The current tweeter will twist out of the mount, and using a screwdriver you can unbend the mesh that is holding the grille in place. this leaves a circular hole with a lip in the tweeter mount. Using one of the fitting options for your tweeter and some double sided tape fit the tweeter to the handle mount as the original was.
fit door card back, wire new mid bass speaker to the wire you ran, attach tweeter to the tweeter wires then refit handle mount.
Hope this helps,