Is it possible to use this tool if running vcenter 6.7 U1 with esxi hosts running 6.5 U2?
Yes. The converge tool only supports the VCSA and PSC 6.7 Update 1. All nodes must be on 6.7 Update 1 before converting.
Keep in mind that if you use other solutions (for example SRM, vROps etc.) using the PSC for authentication in your environment, you need to re-registered these after the convergence is completed.