Otherwise as I've said before, I'm quite a big fan of the surface weapon system, its nice to see X-COM decided to actually lookup decent weapons instead of bringing harpoon guns. I still wish there was some kind of early game surface armour, even just for the cosmetics since its a bit amusing/comical seeing soldiers marching to battle on the surface in diving suits.
As this takes place after X-Com: UFO Defence, Personal Armor would make sense.
Even if Elerium is too expensive, Personal Armor just takes Alien Alloys, which are easy to make. Since they are apparently better than steel, they should even be commercialy available, so it Personal Armor from the first game could be made without any prerequisites.
However, to explain AA are not used to make protective suits, Personal Armor would likely have Sonic damage multiplier greater than 100%. 200% or 300%.