Hello,
Spot directing would help you solve all of these questions.
With spot directing, you can have them walk from anywhere to anywhere. You can also make them shorter or taller with spot directing. And for fitting more than 3 people on the screen you would use spot directing to size the characters small enough to fit everyone on the screen. 
At the bottom of that link thread, there’s also a link that then tells you how to use spot directing and walking. But first start out with getting familiar with the helpers tool.