Normally no, it would be more concerning if all would be running on the same server.
It’s quite unclear what kind of app you mean, for prototypes almost everything is fine. In production you can have totally different requirements based on the need for performance, scalability, security etcetera.
But still, separate servers or even providers is the way to go. What is interesting though its that you might want to keep them in the same data center. Compose offers specific AWS and DO datacenters: https://www.compose.io/aws/ https://www.compose.io/digitalocean/ so you can choose the right one.
No in general not. Exceptions can be very heavy computations which involve lots of database activity. Think of complex equipment loggings thousands of measurements per second. For web/app like usage there are not many of those things to be found. For sure it won’t be a bottleneck in the beginning.