If you're doing Oly lifts, I'd stick to 1000mm wide on the platform.
You can get 900mm racks but they're generally lighter gauge (not that I think the average user would break one at home).

But weren't you planning on a plywood base and then build up on that? So even with a wider rack, it will go thru the rubber but in to the ply base under the matting. Sounds complicated to get it all together though, hence why I would just bolt it to the concrete...