Media Composer does not separate sequence type from project type like other NLE do, (even Avid DS). The project is the sequence type and will be for every new sequence created within that project. If you want a sequence of a different frame rate, then you need to create that project first, then open the bins from the other project and proceed. From this perspective, you can mix any sequence type with any sequence type, but think of it as project to project.
As you’ve noticed, within a project, you can change resolution, but not frame rate. You can change project from SD, to 720, to 1080 but the frame rate remains the same. This is true for most cases except for 23.976 NTSC and 24 NTSC and perhaps PAL but need to refresh my memory again.
Jumping between 1080p/23.976 and NTSC 23.976 will change the START and all AUX TC columns to count as 30 with pulldown or native 24fps. There are historical reason for this, but that too could use a refresh since the introduction of HD.
Michael