Can I charge my new iPhone with iMac

Recently I bought an iPhone online and it's delivered to me today. I don't have a charger brick with me and the charger I ordered is still on the way and will reach me only by tomorrow EOD, so is it okay it charge my iPhone for the first time by connecting to the iMac 2017 I have, as I don't have any other Apple devices.

Will it harm the battery health if I slowly charge it from the iMac USB out?

Yes, you can charge any Apple Device (or any device that supports charging over USB-C for that matter) using your iMac without danger of damage. USB-C PD (PowerDelivery) is standardised and the devices will negotiate the proper charging parameters.