Your second post has some vital info missing from the first. ie you are adding to a network you don't own.
I have the issue of absolutely and totally cloaking my printer
By this I am guessing you simply mean that the homeowner does not see it on his network.. that is fairly simple to arrange depending on your access to the main router and if it has suitable controls.
So I guess what we need first of all, is the homeowner happy to help you do this setup or give you access to the main router.. ? If so you can use a function called DMZ, or secondary IP. Either of which will allow you to isolate your network from the homeowner.
If you do have access (with homeowner's help or otherwise) tell us make and model so we can look up specs and tell you what and how to configure your Extreme.
For the second part putting the TC in upstairs and setting up the link.. that is relatively easy to do with the warning that wireless links of this sort carry all sorts of problems. I used to have a pair of airport extreme in almost exactly the setup you describe.. one placed above the other on different floors. It caused lots of problems when using clients running wireless upstairs to the upstairs extreme.. then double hop to the extreme downstairs. The worst was using mobile phone on wifi with WhatsApp. Try to talk over the phone was completely hopeless.. yet the link was as fast as possible with that model.. it is just that data (not time sensitive) stuff worked fine so I discounted it as issue.. but anything interactive was woeful. Eventually I twigged by taking the phone downstairs and connecting it to the main router.. so cutting out one wireless hop.. and everything worked fine.
Soon after I did what I should have done in the first place.. run ethernet.
That is why I do recommend using powerline adapters.. if the owner agrees. They are much less of a problem.
I can use an Ethernet to Extreme connection for the printer
A bit lost here.. the Extreme is going downstairs.. TC upstairs.. do you mean the printer will be downstairs.. or it was a typo and you meant ethernet to the TC.
If the printer is full network compatible then use network for sure.. that gets around all the USB issues Bob listed.
I was thinking I could hard wire my iMacPro to the Time Capsule
And using ethernet from the iMac to the TC will also help the setup so you get fast speeds and only one wireless hop.. which works 10x better than using 2 hops.
while having my PC go wifi
Depends on what you use the PC for .. anything interactive like online gaming.. then definitely try to hard wire it.
For simple connection to the internet and data only usage it should be OK.. if somewhat less reliable.
For my wireless link through the floor / ceiling.. I needed to reboot both ends once a week.. but that is with older GEN 5 AE.. if you are using new AC extreme and TC it might work better.. (or worse.. if you have interference issues etc).
Just to explain the problem with wireless is half-duplex.. it cannot Transmit (Tx) and Receive (Rx) at the same time.. half a moment's thought will tell you a transmitter cannot work with a receiver at the same time.
Powerline adapters are full duplex.. it is just they tell big lies.. even bigger perhaps than the wireless people do.
So AV500 for example.. are actually 250Mbps duplex.. and the manufacturer add the traffic in both directions.
Gigabit on the other hand, is 1000Mbps in both directions.. so it would classify as 2000Mbps in powerline terms.
Nor are the max speeds ever realised in real world.. AV2000 is barely faster than AV1000 in most normal homes.