Why are you saying that setting nonce to a random value will prevent other nodes connecting to your private net? According to the docs, nonce is 64bit hash that proves that sufficient amount of computation has been done to comply with proof of work algorithm. Genesis block does not have prof of work because it is the first block. Logic suggest this statement is incorrect, because, to read the genesis block, anyone would need to connect to your node first. So, why are you saying that changing nonce will prevent other nodes to connect?