The callipers used in that disc brake conversion shown are the Cinquecento type and using Fiat 126 stub axle assemblies. They have a 3 bolt mount to the adaptor plates as opposed to 2 with the 500 ones.
The calliper pistons are approx 46mms diameter. I suspect that the conversion shown is only suitable for 13" wheels unless they have ground off some of the outer calliper casting or had special discs and hubs made. If you can run with 13" rims and have the correct adaptor plates you can put together a disc brake conversion using the standard 240mms diameter discs for about 100 pounds. The smallest Fiat disc is 227mms diameter , also unsuitable for 12" rims plus has a different offset to the 240 which presents other alignment problems. I am currently testing a set up using a reduced diameter 217mms disc which is the smallest you can go without modifying the calliper, still not small enough for 12" rims. The off-the-shelf conversions for 12" rims , where they use fiat parts, tend to use 227 discs taken down to 210mms.