Not an expert, but as far as I know running with mismatched drives isn’t bad - it’s just a pointless long-term configuration.
Are you planning on mirroring the drives? If you upgrade to 4 drives in the future you could just do 2 mirror vdevs with mismatched drive sizes per vdev. Also I’m surprised you’re bothering with 6TB drives - you can get 14TB very cheap nowadays (recertified).