Mixed SQL Server instances with static and dynamic ports Let's assume someone has 5 different SQL Server instances running on a server. Initially all are using dynamic ports.

Is it safe to restrict one of those instances to use the static port 1433 while all others still use dynamic ports?

Normally, depending on startup order it could happen that one dynamic instance claims port 1433, so that the fixed port instance can't grab it later on. Would this possibly prevent the fixed port instance to run?

Or is there any internal mechanism in SQL Server that starts fixed port instances first, and dynamic port ones afterwards?
When a SQL Server named instance configured with dynamic port is started, the dynamic port is provided by windows. The dynamic port range is outside the range of well-known static ports, where port 1433 resides. The dynamic port range is between 49152 and 65535, as documented here.

At subsequent startups, the instances reuse the same dynamic port used during previous startups (the port is written in the registry). If the port is found to be in use, SQL Server asks the OS for a new port. This behaviour is documented in this page, in the "Dynamic Port Allocation" section.

To answer your question in a single sentence: the scenario you describe is impossible (in a supported version of Windows with ranges properly configured).