First things first. I've never built a robot before and doing a google search yielded this forum. I have a B.S. in Physics but that doesn't necessarily translate to being helpful with robotics, except for being able to think outside the box and do math.
Ahemm... In my experience, the more schooling => the more inside the box - it's the untrained, who doesn't know there is a box in the first place, that transgresses the walls (unfortunately much too often in fruitless directions)
There are quite a number of tutorials on this site that should help you get started.
As the title says I want to build a beginning robot. I want to make a 6 wheeled robot
Doesn't go well with differential drive and if you're planning doing odometry, add a seventh trailing wheel just for that purpose.
I'm looking at making a 6x12" robot [...]
6" diameter wheels
So, the middle wheel will be touching the surrounding two wheels?
I realize with bigger tires you can't carry as big a load
Wrong. Tire size (diameter) has nothing to do with that.
You need to view the transmission as a whole, from the rotor of the motor to the tire on the ground.
Wheel diameter only changes the needed gear ratio.
1. Where do I start to estimate: Weight, Voltage, Torque, Acceleration, Gear Ratios, etc etc? Most say to guess but this isn't good enough for me. I need formulas or some way of estimating. I want to avoid over weighting the first robot so cruising velocity can be drastically reduced, since this will mainly be tied to dc motor weight x6, for fundamentals and functionality.
As mentioned, plenty of tut's here, but B.S. or not, this is the real world, so prepare to hone your guesstimating skills. (Or does the B.S. just stand for what it usually does
2. What is the recommended dc motor for robots? I'm thinking brushless design, as stated above, but in reading up on them they need a special controller. I'm thinking High torque as I want the robot to be fast.
Brushless equates to the most power in a given packet size AND to the most involved driver.
Since you haven't mentioned your skill level in mechanics, electronics and programming, I'll advice you to go with regular brushed DC motors, either with integrated gearheads, or with an external gear reduction.
3. Recommended chassis material? I was thinking plate aluminum as it's light but a plastic would work as well. It needs to be able to hold up to a beating as it will be tested outside and plastics can be rigid and crack under the strain of a vehicle not having a suspension.
Plastics is a very wide area and you can get just about any specs you want. Some plastics are very heavy though.
For the Non Plus Ultra design, choose carbon fiber and aramides.
For the cheap experimental platform, plywood is my preferred choice, as it allows faster changes than aluminum, takes less tools (easy to carry what's needed for changes on location) and is much cheaper.
4. Recommended voltage? I'm Thinking 24V as I'm looking at a 6-12V Motorx6
I'm confused. You have mentioned up to 36V (which IMO is way beyond sensible with NiMH).
Just hits me... You need to study basic electronics - 3 separate 6V motors takes... 6V to run...
I have a feeling you added their voltages to 18V?
Go with max. 12V for starters.
5. If a brushless motor is used what micro controller is recommended?
The choice of motor topology doesn't dictate a specific microcontroller, but you'll need a (quite pricey) brushless controller for each motor.
If you, in reality, just want to make a ROV (a glorified R/C car) rather than a robot, you don't need a microcontroller at all - just build it like any other R/C vehicle.
Your post is a bit self contradictory in places... You want it to be very fast, but speed doesn't really matter and so on. Better sit down and sketch some rough (but consistent) plans, refine and distill a few questions out of that for posting here.