1. I usually have my sim send their employees to train every day. As they get better/higher in ran I do the more expensive training options.
2. I stick with however many are necessary to keep the guests satisfied. Most of the time this is one. On rare occasions, I'll have 2, but definitely no more than that.
3. I promote when I think they deserve it. When they get to lvl 3 in their skills I boost them to rank 2, and usually around 5 for 3, 7 for 4 and max for rank 5.
4. This seems to happen on occasion with employees. I just fire them and hire someone else.
5. Sometimes waiters will just stand around on their phone and do nothing. Usually this occurs at lower levels. I criticize them and make sure I send them for training. If they still do nothing, I fire them and hire someone else again.
6. Typically my sim will welcome everyone, then prioritize food once they've ordered. After that I always check on them once the food comes. I'll give free drinks if someone's food gets spilled or if the sims don't leave after getting their food which seems to be a glitch. Keeping staff happy is usually easiest by promoting regularly.