I have a 512GB SSD M.2 (I think PCIe) installed in my 3510 and need more storage space. 

I see the following specs in the manual:

Feature                   Specification
HDD 2.5 inches     Up to 1 TB
SSD 2.5 inches      Up to 360 GB
SSD M.2 SATA       Up to 512 GB
SSD M.2 PCIe        Up to 1 TB

It does not seem like I can install a SSD M.2 SATA under the M.2 PCIe, is this correct?

With the M.2 drive installed, I followed this article on installing a drive in the 2.5-inch bay: https://www.dell.com/community/Precision-Mobile-Workstations/Precision-3510-2nd-hard-drive-part2/m-p/7305362

In which case I have to install a M.2 drive there. What are the size limits for it? Can I increase this with a BIOS update? 

Thanks for any help