No, you don't need to bring your own clubs.  They will have you hit a few shots with a stock club as they get a feel for your launch numbers, clubhead speed etc.  Then they begin to change change shafts out and take other measurements to see any adjustments that would need to be made to loft and lie.

As for the midsized grips they can pretty much do any combo you need.  I just got mine fitted with standard size grips with two wraps of tape to make it a little thicker.  

I feel you must have your own clubs. Getting a baseline with your current equipment is essential in determining whether new clubs are needed and what improvements are achieved with new clubs. Without that I feel like you are just selling clubs and not helping the player improve.

I would bring your own clubs. It will give you an idea with your numbers on your current clubs compared with the new ones. I just went through the Titleist fitting and it was unbelievable what I learned about clubs and the differences with shaft, swing weights, etc. I would highly suggest going through their fitting process. Definitely top noptch!!!!

I went through an iron fitting a year ago and through a driver fitting a few weeks ago.  In my opinion you do need to take your clubs with you.  It's improtant to have them to develop a base line from a numbers (launch angle, spin, carry, etc and a feel perspective.  Once your fitter has that base line you hit every combo there is until you find a fit.  Pretty cool to see all the numbers.  Good luck