Is it harmful to say "On the authority of Abu Hurayrah, may Allah be pleased with him, that the Messenger of Allah said" instead of "On the authority of Abu Hurayrah, may Allah be pleased with him, he said: The Messenger of Allah, peace and blessings be upon him, said"?

There is no harm in that, because the meaning is the same. However, it is better to adhere to the wordings narrated by the Companion and not intentionally alter them. Indeed, Imams Ahmad and Muslim were meticulous in the wordings of the narrators, and Al-Khatib mentioned that this is by way of recommendation, not obligation.