Yes, you can but if would be better, if the older person is not even the boss, just call him so, 老闆. For a lady, 老闆娘. You get better service. Here we are talking about a person working in a small shop or restaurant.

No, that's for the waiters in the food-related establishments only.

I often just call them 你好，than ask question. 服务员 usually be used in restaurant or coffee.

yes, you can also say “售货员”

You should better not call her as 服务员 This expression is usually uesd in restaurant or in other service industries. Sometimes it can confuse people.The best way is to say 售货员.It is a general expression. If you in a small shop,you should better say boss老板