Should use no coolant.
Quite often they develop very small leaks from the water pump, the water rail O ring, or the water rail itself (tube that runs along the front of the engine block). Generally the only way to spot these is to bring the car up to temperature and crawl under it with the engine running, look upwards. But if you can't see anything, compression test.
The pulsing idle can be an air leak, but sometimes just means that you need to clean the throttle body. Get hold of some cheap carb or brake cleaner (often poundshops or Netto/Liddle/Aldi will have some). Clean the outside, then remove the throttle body and clean the inside.
These are easy cars to work on, so don't be scared of DIY. The Haynes Cinquecento manual is useful (the engines are very similar, the chassis nearly identical, and the Haynes is easy to find your way around) and the Seicento Technicak Manuak is a 3 part download in the "Downloads" section here. You need a (free) program capable of opening .rar files. Google will find.