you don't have to plug the two processors. I have the Tyan i5400xt (Xeon Harpertown) and I simply plugged the first processor for months. You don't have anything to do, the bios will see that there is only one cpu.
Sure it can.
My suggestion is: Don't do it. Fill it up to the limit right away.
My argument is that in general these kinds of system REQUIRE all CPUs to be 100% identical.
If you wait a month too long you may very well run into the required additional CPUs no longer being available.... of being very expensive.
Only desktop Intel/AMD stuff. On other CPU systems like Itanium you need dummy voltage regulartor modules that are put into the CPU socket.
I once (around 2003) bought a MP Board for AMD and plugged in only one CPU. Thought spend 200 Euro more and therefore have a later option (when CPU's are cheaper and my business is going) i can get double speed upgrade. Didn't work at these days as the CPU was quickly out of sale and no way to mix CPUs with different speeds.
I learned from this. Never do to provide yourself with an upgrade path. Buy now and sell it after upgrade - this saves much more money in the end.
I have 2 HP servers here with 2 CPU socket available and running with only one CPU for 3 years now and there's nothing wrong about doing that plus it give you the possibility to add another CPU if needed at lower cost later ;)
Yes it is possible to run an empty slot. But that is an issue for the chipset that manages the onboard operations of your motherboard. The IBM P570 that I have runs with optional chips off but in place. If you want to "buy" the chips you have IBM will send you a code to activate them. You can also have the system shipped with only the chips you paid for and have nothing else in place.
Same is true for HP, which I moved off of recently.
Sever companies love to be able to sell options but if you get into the grey box space it is hit or miss.
Or what about using all dual core cpu, and then if needed, upgrade to quad? One virtue to that is more memory bandwidth, at the cost of having your memory fragmented (each socket has it's own memory bus and sockets) and perhaps needing to replace memory in the future.
Why don't you consider buying a lower-end motherboard with enough processors to fill all slots? When you need to upgrade you can order a quad-cpu board along with matching processors and either delegate the smaller system to other tasks or sell it second-hand.
You'll save some money upfront, get a matched system when you need it, and also possibly offset some of the depreciation on the smaller board / procs with savings on the bigger board... tech gets cheaper quickly