Unfortunately they are rather unique. Fiat likes to use uncommon thread sizes for lots of things, this being one of them.
Replacement nuts (for the ones you will damage when taking them off) are available from one or two sources (there are a couple of threads that discuss this). And a couple of the Fiat specialist vendors offer SS brake hoses with the needed size of fittings as direct replacements for the X1/9 (at a price). Thankfully the hardlines are not unique, so you can replace them (if necessary) easily. But they do require "bubble flaring" (European style ends) to use the special nuts. So you will likely need to buy the appropriate tool to do those flares.
There is another option (there always is); you can use more common fitting sizes with adaptors. The adaptors would connect to the calipers, then regular nuts are installed on the hardlines (with regular flares, no new tool required). To install the common style nuts you can cut off the very end of the line (old flare), slip the nut over the tube, then use a common flaring tool to make a new flared end. Then it is easy to get SS flex lines to fit the common fittings on both ends. A bit more work, but actually less costly compared to all the "special" stuff.
Regarding the issue of removing the stuck fittings. That is a very common occurrence on these calipers and the masters. Typically the nuts a sacrificed (destroyed) in order to get them free. One frequent tool used to do this is vice-grips. Personally I have always managed to get them free with heat. But admittedly it takes quite a bit of torching to get there. I recently bought a new tool that might help with these frozen nuts. It is a box wrench with a open slot (to pass over the hardline), then a bolt clamp closes the box end tightly around the nut. I haven't tried it yet so can't report if it helps or not. If you know you will be replacing the flex hoses then you can simply cut them, take the caliper to your work bench, clamp it in a large vice, and use a long breaker handle on whatever tool you use to engage the nuts. Also makes it easier to get all sides of the fitting's bore hot with the torch.
Use some anti-seize during reassembly to avoid this mess in the future (and it will need to be done again, seems Fiat brakes require frequent servicing compared to other makes).