I added a brand new node into the private network, without bootnodes. Over time this node DIDN'T seem to add any peers to talk with. Is that normal and expected?
Yes it's normal and expected. The P2P protocol requires that you indicate at least a bootnode from which your discovery will start. Bootnode is a venue for new nodes otherwise they can't know where to start searching for peers. There's a list of default bootnode in clients, but as you run a private network, this list isn't useful for your. Depending on the client you use, there may be different ways to turn a node into a bootnode. If you indicate your client and I'll edit my answer.