another question related to this, since we are already discussing about segwit address.
If one private key can generate a P2PKH address and a segwit address, when you spend from the P2PKH address, will the spending also reflect on the segwit address? I mean you are spending from the same private key, which hold just one amount of bitcoin right?
That's not how it works. Read up on UTXOs. The wallet won't let you mismatch script types for a given private key anyways.