It seems like you're asking a few different questions here.
After Effects uses a pixel-based coordinate system, which is not how Maya handles 3D space measurement. You can set working units based on real measurements in Maya (see pic), however, transformational increments (as well as all other adjustable values) are set to Float in Maya. Having your animation lining up with an imported .ma file will depend on whether or not you baked your camera data, and how you have your film settings applied to your camera. Overall, comparing the two coordinate systems is like comparing apples and oranges. While both fruit, they're pretty different.
As far as collision detection for a Particular emitter parented to transformations of imported Null geometry from Maya, what are you expecting them to collide against? Rendered imagery from Maya? That won't happen. Particle collisions with Maya geometry need to occur from within Maya. Are you trying to have the particles collide with geometry that exists within AE as 3D layers? If so, have you set those layers as collision layers within Particular's effect controls?
1) I have baked the camera in Maya, and everything lines up nicely, as long as I parent the position of 3d layers in After Effects to one of the nulls that I imported from Maya. If I copy the position and/or anchor data from one of the nulls to a 3d object, it doesn't work. For example, the center of Maya's world is 0,0,0 as postion. the null I imported at 0,0,0 shows the position in After Effects as 0,0,0, but if I copy that position directly to an After Effects 3d layer, it doesn't line up. If I use an expression to tie the postion of the After effects Layer to the Postion of the null from at 0,0,0 it works out. I figured it was that two programs calculate cooridnates differently. For example, isn't AE Y switched +/- from most other programs? and new objects created at the center of AE's 3d world usually come in with positive values on the x,y axis' and only the z at 0. Correct me if I'm wrong.
2) I did create a floor layer in AE, I know that AE layers won't interact with Maya layers. I've set the bounce up a million times, and it didn't work right with the camera I exported from Maya. I tied the emitter postion in Particular to the position of a null I created in Maya, shows up in the right spot, looks great. I created a 3d solid and tied it's postition to the 0,0,0 null I exported from Maya. It looks like it is in the right spot and travels with the camera correctly, but the particles don't bounce (yes the blue postion arrow is up). I thought maybe it had something to do with my 1st question, i.e. tieing the postion of the floor layer to the null with Maya's 0,0,0 position isn't actually bellow the emitter in AE. or something...
Does that make more sense what i am asking? I'm just trying to see the limits/differences in how the two programs interact.
If I copy the position and/or anchor data from one of the nulls to a 3d object, it doesn't work.
I think this is because the camera shape attributes that are imported from the .ma file help construct the the AE-based universe for the referenced Locators created in Maya's world space. This differs from how AE would naturally create the 2.5D environment with an internally created camera. Why doesn't copy/paste work? I'm not sure, but I'm guessing it's because of the fundamental differences in how the two different programs differently interpret space.
For example, isn't AE Y switched +/- from most other programs? and new objects created at the center of AE's 3d world usually come in with positive values on the x,y axis' and only the z at 0.
AE starts measuring at the top left corner (except for Z space). If you enable rulers, you can see that 0,0 is top left. This is different than traditional 3D coordinate systems. If you adjust your anchor point to be top left, your XY coordinates will show as 0,0.
I tied the emitter postion in Particular to the position of a null I created in Maya, shows up in the right spot, looks great.
You probably already did this, but did you tie the z-position of the emitter to the z-position of the null? Is the floor big enough to catch all of the particles for collision?
Yeah, I did tie the emitter to the z postion using the proper expression. You can also set the floor to be infinite, i tried that and making the floor huge. I have a workaround/reconceptualization of what I'm working on to get around it. I will try to email that folks at trapcode directly and see what they say. Thanks for the explanation of the grid/difference. It's what I assumed, but it was nice to get it nailed down.
I was able to fly in on the side view, and the solid that I parented to at MAYA 0,0,0 to be the floor, was way down low on the Y axis, even thought it looked right with the imported MAYA camera. not sure why or how, but it's working and that's all that matters, right?
except the bouncing particles don't look in the right perspecitve. I'm getting closer though....