I'm a newbie to OpenGL, so here it goes: How do I remove one fragment shader stage for another? Would I 
just use glUseProgramStages(...GL_FRAGMENT_SHADER_BIT...) on the new fragment shader and would it 
then remove the other one? Can a pipeline have more than one fragment shader in use? :/

Like this?

Draw call:

glUseProgramStages(vert shader)

glUseProgramStages(first frag shader)

draw stuff;

glUseProgramStages(second frag shader)

draw stuff