Adding more speakers in front door

[dkahns51]

I currently have a set of JBL C608GTi-mkII components. I was wondering when adding more speakers to the front door does it matter what type i get? I dont want another component set, i just want another set of speakers. Do i need to go with the same brand? Do they need to be the same size? What would be a good compliment to the JBL's to make the mids highers.