Well more Aliens will only make them Panic faster.
There are two things which greatly imbalance vanilla gameplay:
- Psi
- Economics (Laser Canon CashCow)
I changed those factors in my mod significantly, so you have economic constrains, since maintenance cost and wages are pretty high.
PSI can still be unlocked relatively easily, so you con build PSI-LABS to screen you soldiers, but to use PSI you need PSI-Amps and those need some extra investment into research.
Simply adding more Aliens will only increase an overflow in cash, proper balance is needed. Took me actually a while to achieve it.