The formation as shown on the tactics creator should be how your players line up in the defensive phase. Therefore, your choice should be a 4-4-2 or 4-4-1-1. In order for this to shift into the 3-5-2 or 3-4-3 in the attacking phase you have to select roles and duties to facilitate this change.
For example, looking at the movements you'd like your back line to make, I'd suggest your RB as an IWB(D) or FB(D) and your LB as WB(A) or CWB(A). The RB will theoretically stay back and narrow as part of a three man line with your CBs. The LB will push into the midfield line to provide width on his flank, or even advance to the byline.
I had a quick play on an FM Touch test save I have with Bayern to find some basic examples of how duties affect movement between phases. I don't know enough about Fiorentina to offer suggestions for which roles you'd actually need to mimic their real life tactic, but these principles should help you discover the positional shifts between the defensive and attacking phases. I tried to pick roles that are similar to the ones outlined in the articles posted in the OP. The simple 4-4-2 in the tactics creator is here:
These examples come from a Champions League group stage tie against CSKA Moscow, who play in a defensive 3-3-2-2. Here is an example of build up in the defensive third. My RCB(D) Boateng has the ball in the centre of the pitch flanked by Kimmich at RFB(D) and Hummels at LCB(D). Alaba has moved forward into the midfield line creating a 3-5-2 shape. Alaba and Gnabry are pinning CSKA's wing backs deep early in the build up:
The next example is deep into the attacking phase. Again, the three man back line is holding position. James, the left sided IW(S), has pushed into the box to support Lewandowski and Muller, whilst Alaba and Gnabry maintain the width. This is a clearer example of 3-4-3 appearing:
Finally, here is the line-up straight after a CSKA kick off. You can see we are lined up in the basic 4-4-2 shape:
I hope this helps!