Aurora R8, spec questions, mix memory sizes, disk > 1TB, VT)

I have been working on an almost 8-year-old PC which despite its age, has been serving me well with 16GB RAM, Windows 10 Pro, 1 TB SSD and a i7-3820 CPU.  But for the last couple of years, I have been working heavily with robotics, simulations and other software that requires extensive use of virtual machines and the i7-3820 does not support virtualization which means I cannot run 64bit VMs, which means I can't use the system for my work.  Big deal breaker.

So I decided to bite the bullet and get a new system, so after a bit of research I placed an order for an Aurora R8 with the following specs: Windows 10 Pro, i9 9900K, 850 Watt PSU+liquid cooling, 1 TB M.2 NVMe SSD, 16GB (2x8G) 2666 RAM and GeForce RTX 2070.

For various reasons, I could only place a single order below a certain monetary limit but have additional funds to spend on the system and my intention was to add memory and a SATA hard disk for data storage following the initial purchase.  I don't really care about overclocking so for memory, DDR4-2666 will be fine, I just care about the amount so I can run the multiple VMs.

As I was doing research on these components, I run into a few things that worry me a bit, specifically:

1)  It seems as if upgrading memory is a bit of an issue due to various proprietary issues so I plan on ordering memory directly from Crucial, specifically their kits that are guaranteed to work with the R8.  My question is, do the memory sticks have to be all the same size?  The manual on page 13 indicates that total memory can be 8,16,32 or 64 GB at 2666MHz but does not specify if 32GB can be accomplished by using 8G, 8G, 16G.  As I will eventually go for 64G, I don't want to buy 2x8GB sticks which will later be disposed of when I upgrade.

2) I saw a post of a user having issues with virtualization, specifically running Oracle VM VirtualBox.  Can someone confirm that they successfully installed and run a 64bit Oracle VM on this system?  (This is a deal-breaker for me, it's the only reason I am upgrading my current system and if running a 64bit VM is not possible, the system will be returned unopened).

3) The manual on page 16 indicates that the 3.5-inch hard drive can have up to 1 TB capacity.  Is this an artificial or other limit of the BIOS/motherboard?  I would like to add a 4 or 6 TB and don't understand if this is an actual hardware limit or just a limit on what Dell offers with the system.

Any help or suggestions would be highly appreciated, especially regarding the ability to run the Oracle VM.