New Build 1000x600x200mm (newbie)

[JAZZCNC]

Well with the Y axis rear plate(across gantry) you have room to have it wider than the front plate because it will over hang the bearings which the front plate can't or will crash into bearings. This will make Y axis much stiffer and be easier to access the bearing bolts. Yes put the bearings on the rear plate, rails on front moving plate.

For the Z axis you want hi-win HGH20CA Z0 which are the slim type and for the other Axis you want HGW20CA Z0 which are the wider bearing and will give a bit more support. Don't use 15mm the bearing blocks are too small and fiddly making building a Z axis more difficult due to clearences etc.

With the design I feel you are a little low on the height as it won't leave much room for cutters but then again I don't know what your cutting so may not matter.
Neils suggestion about X bearings looking poor is valid and has it is will need some bracing to help stiffen which is easy enough. But there's another change that I would make which will lessen the cantilever but more importantly allow for easier adjustment of the ballnut and allow some protection for the screws.?

Make the Gantry sides drop to the inside of the ballscrew not the outside.? This will shorten the gantry width and reduce bearing plate width and cantilever making it stiffer.
It allows access from the outside to the ballnut for adjustment and greasing etc.
Another important and often overlooked thing is screw protection, Your way makes it harder to protect the screw with a cover because of the gantry side being on the out side. If it's on the inside if the screw you can make a cover using C channel with a brush strip on top that covers any gap and wipes the gantry. This makes a very easy way to cover and protect which you'll want to do if cutting aluminium or wood even with screws on outside. (see pic)