So one question that I do have is; Has anyone put 2 traction wheels on opposing corners of the base and have omnis for the other 2 or would that effectively make all 4 wheels traction wheels?
I’ll highly suggest going for an X drive with mecanum wheels oriented 45 degrees from each other
I’m actually being serious with that question but I should definitely rebuild.
I asked that because we have a geared base (it’s a 4m 5:3 H base on 3.25in omnis) so we can’t get 2 drive wheels in between the main wheels (6 wheel drive isn’t possible) and having traction wheels on opposing corners would move the pivot point to the middle of the bot and not the back (would make turns faster)
Given robot base of (A B is the “front”)
A — B
C — D
Omni on A & B with traction on C & D is fine as is the mirror image of traction on A & B, omni on C & D
Omni on A & D with traction on B & C will have less turning ability. The traction wheels still need to slide on the turn,
Omni on A & C with traction on B & D will have the same turning issues.
I have seen Omni on A & B, mechanum on C & D and while it drives, the spin is not in the center of the robot. But it seemed the spin was a little easier. YMMV on that configuration.
The final case of Traction on A & B and mechanum on C & D doesn’t spin well, again because the need for the traction wheel to slide on the turn. Need to make sure the rollers on the mech make the bottom part of the X so they are working with each other.
Other corner cases of using a different type of wheel at each location is an exercise left up to the reader. 
Ok thanks i’ll probably try a few things out but we definitely don’t have room to put mecanum wheels on it.
(I mainly asked because we are trying to implement odometry and some sort of (side to side) traction would definitely help) (also for defence if we ever go to an in person comp)
I never would have thought of putting mecnums and omnis on the same base
I’m gonna slap a chassis together later and try that out
You are going to also need to slap some software together. 
I don’t see why you would ever want 2 omnis and 2 mecanums. it won’t be able to strafe, and mecanums that can’t strafe are pretty much pointless. if you want to resist pushing but still have decent turns with a 4 wheel drive, have traction wheels or locked omnis in the front. it will move your turning point towards the front of the robot, but usually this isn’t a bad thing.
Related to this, I can see that on this robot they have 2 mecanum wheels in the back, and 2 omni wheels in the front. ( in my first year of vex, as an IQ team, we called omni wheel Ninja wheels. 
) I’m not sure why, because, according to my knowledge of mecanums, that will not be omnidirectional. I’m not sure if it will even drive well.
Also, you typically want to put the traction wheels on the motor driven wheels, it you only have 2 motors driving your base.
actually, this robot had 4 mecanum wheels in the back. it was because they didn’t have room for a mecaunum wheel in the front, so they compacted their 4 mecanums in the back and used a free-spinning omni in the front as a sort of anti-tip.
this robot could still strafe, but I don’t think nearly as well as a normal mecanum drive.
Okay, that makes sense. I did not notice that.
would it be possible to make a 2 motor mecanum drive, still omni-directional, maybe by connecting the wheels through gears?
Yes. Just have 1 motor for each pair of opposite corners and that should have enough states for omni directional movement
no because then you cannot turn.
Oh you right. I guess you do need a ratchet setup and power losses in certain directions afterall. But hey I did give a solution that does technically have omni directional movement.
2 motor holonomic is not possible. Your best bet is a kiwi or h drive with 3 motors.
Edit: technically possible to have a hilonmic drive with 1 or 2 motors but it is so incredibly inefficient that I am not going to consider it. It would work by having one motor spin direction select what mode the drive is in and the other to power.
You can make a strafing drive with 2 motors quite easily. Just have one motor control the x-axis and the other the y-axis. You could also turn by off setting the x-axis wheel and y-axis from the center and then combining the y-axis motor spinning to make it not turn and instead strafe.
You could also do this with one motor by using a differential but you could only strafe in the x-axis is one direction and likewise for the y-axis. It would also be impossible to control if your turning or not.
Edit: Forgot to say you has to offset both wheels
This set of posts is actually a good description of what the possibilities are. 2 motor holonomic drive? - #3 by gusepina
This is why searching for past threads is a good idea. Lots of back history worth digging through.
1 wheel 1 motor diffy swerve with lexan antitips on all sides