If I could have a Qnap as 2 node, would this harm my main node?
Every node added to the same network will reduce the chance to be selected for all nodes in the same /24 subnet of public IPs: we filter out all such nodes and select only random one for uploads for every segment.
If you add a new node, it will reduce the chances even more, because each new node will going through a vetting process. The vetting node can be selected for uploads only in 5% cases until got vetted. To be vetted on the satellite, it should pass 100 audits for it. For the one node it should take at least a month.
For multiple nodes it can be in the same times longer as a number of nodes behind the same /24 subnet of public IPs.
Since you want to run a two new nodes, the selection of each of them is 5% chance, next we filter out nodes, thus your old node can have a noticeable reducing of incoming traffic, as much as 5%/3 ~ 1.67%
In such situation I would recommend to do not start any node until the old one would be almost full. Then run only one and wait until it will be at least vetted or better - almost full, then start a new one.
When a node is full what should we do? Increase capacity? or add a new one? Which is more recommended.
A node with Qnap or with a PC is better.
Thanks for all this help Alexey
Depends on your case. If you have a RAID already - you can expand an array.
If you do not have - you can start a new node. Since the previous full it will not reduce the chances of selection for the second node.
PC is faster, but consumes more power and usually more noisy 
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