noob questions
 

if i buy lowering springs (or coilovers) and take them to the shop to install with my new wheels, do i need to tell them exactly how much i want it lowered?

i.e. can i just tell them i want it "to look flush with minimum wheel gap" or do i need exact inches?

Specific instructions are the recipe for customer satisfaction.

If you aren't sure, give them as much info as possible and let them know you'd like to check before they are done. Don't forget it will settle a little after the install.

If you buy lowering springs you can't adjust them so the answer is no. As for adjustable coilovers, yes you can change the ride height. Just let them know how low you want it and they will dial it in.