1. You can mate "concentric" for two spherical faces, if you make a spherical sliding bearing. Click "mate", "concentric" and both spheres.
2. You can mate "tangent" for two spherical faces. It is necessary to add mate "distance" between origin of one part and some planes of other part. You can clickan origin in design tree.
3. You can use a mating of origin of parts, if this points coincident whith a center of spheres. Click "mate", open parts in design tree, click "origin" of both parts. You can use mates: "coincident" and "distance"
4. Do references points in PARTS, which have spherical face:
Reference geometry => Point => center of face => click on spherical face. This method is applicable, if you have no origin in center of spheres.
Use other faces or planes for additional orientation.
Other mates are used very rarely.
Also, when i right click on a part i only ever seem to have the option to FIX not FLEXIBLE
If you insert assembly 1to assembly 2, parts, which move in assem 1, cannot move in assem 2 default. Right click on an assem 1 => Component property => Flexible. Parts have "FIX" or "FLOAT" only.
I will be able to help anymore, if you will expound aproblem more concrete.
Ex