Well 6mv would definitely be too much then?
No, 6mv won't be too much. It will be noticeably different than something like 4mv, "louder" at the same volume setting but most preamps set within a 36dB to 43dB (MM range) and will handle quite a bit more before distortion or overloading.
It could even have 50dB of gain and still be okay, but it's getting close. Depends on how much gain your pre-amp section is set at if you can get the specs for your 6100; I looked for it and saw that the min. level was 2.8 mV but that may have changed over the years. But basically, no it won't be too much.
If you use a separate preamp, make sure it goes to a line level input, not to the phono input. That will definitely be too much.
As for set up, learn to do it yourself. Get a digital scale cheap off the web, like $9-12 for an inexpensive one, and use the Technic's overhang tool or use a set of calipers, or a small mm rules/protractor and set it to 52mm from tip of diamond to the back of washer (flat up against the stop/boss). Look at the post above and it gives the exact diagram. Make sure your tt is level. It is easier to see with a good tt level. Don't know about your specific TT and if it has a strobe, but there's an app you can get for your phone to check TT speed & wow/flutter. I had horrible sound once with a TT that wouldn't hold speed. That's easy and obvious but sometimes forgotten.
You can get away with that 52mm as long as your cantilever and cart body are straight in the headshell. I find the main items to look for are the VTF (weight) and the overhang set properly to get decent output. This all relies on the cart being reasonably straight in the headshell and reasonably close VTA. A proper protractor model (downloadable as pdf, printed actual size not scaled) will get you most of the way there but make sure you are using the correct one for Technics.
Sometimes you have to adjust for azimuth and making sure the cartridge isn't tilted inward or outward from the groove (left/right inwards or outwards towards spindle as you face the cart). Some headshells and tonearms don't allow for that, I'm not that familiar with yours but someone here will know. I've had some noticeably bad headshells/tonearms on used turntables that had to be adjusted.
You can get close by just looking at it, and even closer by using a protractor. Look up some instructions off the web on using one and setting up your cart. You don't have to spend $$$$ on equipment as your ears will let you know and you get more and more comfortable with set up.
It's a learning curve. I like the suggestions of using an inexpensive cart to start out with in case you damage the stylus. Maybe buy something second hand on the cheap and a spare headshell to mess around with? I would recommend that.
I had an old JVC LF-41, AT cart powered with an old AX-2 amp set up by a dealer back in 1980. Thought it was great. Finally learned how to set up on my own and found the overhang was way off. The performance was much improved and my opinion of the AT cart and LF-41 also improved.
You should do a side by side comparison with your P-mount and your new tt when you set them up. Maybe a cheap input switch to compare them back n forth. Your P mount will be set up right and if your new one isn't, you will be able to tell and go back and check.
I like to use a decent set of headphones when doing this as you don't have to move back and forth so much during adjustments and it eliminates how some speakers don't convey trouble as easily as headphones seem to.
Try with VTF and overhang first (52 mm with your Technics headshell) with a reasonable straight cart and see how it sounds. You may be surprised. As you get those used to it, you can start worrying about some of the other items like VTA, Azimuth, gravity during high tides, full moons, solar flares, atmospheric pressure, humidity, and keep your albums clean as well as your stylus.
I used to think keeping my '68 Vette in tune was a PITA, (trust me it was). That was until I bought an old Hitachi 353.
Take your time, enjoy the music.
DR