2. I also want to know how many addresses I can watch until things start to get slow, or worst... the program start to crash?
For Private keys posted on Bitcointalk, I ended up with a 2 GB wallet. That made Bitcoin Core very slow.It's easy to test with watch-only addresses, just get a lot of them, and try it
I expect similar limitations: it doesn't really depend on the number of addresses, it depends on how many transactions they've had.What is it you're trying to accomplish?