Are you using 4.0 or 4.5?
Creating new models requires modelling them, in Hexagon for example, and rigging them. Most people, at least initially, use clothes that others have made though with hexagon and DAZ Studio Pro you have the tools to make the models and to turn them into figures - the thing you lack is some kind of image editor to make the textures that turn flat geometry into something that looks like fabric or hair. I would suggest looking in the Freepozitory forum here for freebies that others have made (make sure they are actual clothes and hair, not a texture set for a set of possibly non-free clothes and hair from elsewhere). Wilmap has a thread with many items of clothing.
Morphs you can make with Hexagon, using the hexagon Bridge, or in some cases within DAZ Studio using the DFormers. There are also numerous extra morphs available that others have made, free and for sale - again, check the Freepozitory.
Yes, you can load multiple characters by double clicking the Genesis base figure in the content library - I can’t recall what the default behaviour is, since I’ve changed my preferences, but if that doesn’t seem to work (check the Scene pane for new entries if you haven’t yet posed the first Genesis) try clicking in an empty area of the viewport to make sure nothing is selected before loading the new Genesis. Saving a character probably saved settings, for the morphs and materials, that will change the current figure rather than a whole figure (though if that was a .dsf file it should load a new figure if you double-click with nothing selected - that’s what the basic Genesis file is).
I’m not quite sure what you mean by mixing different styles - morph (styling) and material (colour) settings for the same hair should mix and match OK, but they will usually be model-specific so you can’t usually take a style from hair A and a material from hair B and apply them to hair C.