I've been told there can only be one of each species on your roster (multiple for forms). The Arcanine I recruited is lv28 so it missed out on practically everything its pre evolution had to offer. I was thinking of just evolving my Growlithe to get a proper Arcanine, but would that mean I lose Growlithe forever? Or does a new mission spawn for another Growlithe?
You would be unable to get another Growlithe. You can only get one of most species, with exceptions made for differences in gender and in form.
Good news though is that Hawlucha can actually teach your evolved Pokemon all the moves they could've learned as their pre-evolution at the level they are at. So you can go and teach your Arcanine moves like Heat Wave or Crunch.