Here's some advice I'd give. If you don't want it, just ignore it
Make the decision about the VOs last (relatively). It's probably a good idea to make your characters aliens but keep in mind that you might be converting (some of) them to humans later. Anyway you've got a LOT of other work to do before you even get to actually finalizing the characters, which is when you will know for sure what you even need in terms of VOs, so it's pointless to think about VOs and voice actors now.
The only exception is reused VO. If you plan on doing that, and especially if it's important to the story, collect your VOs first. Mute scenes don't work very well, and animations can't save you all the time!
Also, do your story first and only move on when it's solid. It needs to be well thought out because it's not gonna help you if at some point you realize you can't pull something off and then you have to scrap a bunch of things and start over.
I wish you good luck and lots of great ideas for your mod