I think you are going to have several problems installing this alarm unless you can find someone who has done one before to advise you.
The Acc terminal is used on most cars so you can have the radio on without the ignition, however, Fiat ignition switches don't have this terminal. From your wiring diagram, it's unclear whether this is essential for this alarm.
Are the 4 relays (3x4 terminal and 1x5 terminal) included with the kit or does the wiring diagram assume they are already fitted to the car? If the former, are they pre-wired and which terminals of the relays (30, 85, 86, 87 and 87a or 87b) are connected to which wires.
I can't see any way the bottom right relay (5 terminal so probably changeover) can work in this wiring configuration. It would appear to have 1 terminal connected to the alarm unit, 2 terminals permanently connected to +12V an 2 more terminals connected to 12v from the ignition switch.
Dave.