Any subspecies of Salamandra salamandra can intergrade when kept together, so S. s. gallaica X S. s. terrestris is certainly possible. I would imagine that most, if not all, Salamandra species can also interbreed - though this is something that shouldn't be done.
I'd suggest keeping all the offspring of this breeding to yourself, as it would be an unfortunate situation if these animals got into the hobby and were then bred with pure gallaica or pure terrestris.
I have seen some individuals from subspecies hybridization, and many of them had colour attributes of both parents and looked quite attractive. I thought some looked obviously hybrids but I did wonder how many would also pass as a subspecies in its own right. You only have to look at the colour and patterns variation of giglioli across its range to appreciate this. One has to remember too that a degree of cross breeding occurs where subspecies over lap.