Well only use password and username.worker if you’re in a pool that requires an account like Suprnova. Those would be to start mining into your account. Otherwise you can just use -t 8 like @cascadiacrypto said.
you will get an equivalent hash rate with 75-100% of cores in use; its recommended to use 75% so it won’t bog the machine down. this is why neqminer does it by default.
I’ve heard that but I don’t think that’s true because I have made tests using all the threads and using default options (75% of the threads) and the results are more or less the same (a bit better if using all the threads).