As far as I understand, 9x18 is an odd version of the 9mm, won't work interchangably, and in all my bullet shopping, I've never seen them for sale. Might want to look into that.
Yea the guy said they arent interchangeable (but a 9x18 will shoot where a 9x19 should be),
Absolutely not. The 9x18 bullet diameter is larger than the diameter of a 9x19.
That wouldn't be a bad option because ammo is cheap. In fact, it might be your best option if you actually want something worth carrying.
Otherwise, the Mosquito is internally the same gun as the P22. The bad reviews are due to retards as I previously explained. If you like how it felt, that's your best bet. I've had no problems with mine, my friend has had no problems with his and if you actually read the reviews on the SIG forums the ammo issues that cause the complaints are explained.
Stay away from anything in .380. As someone else said it is THE hardest to find ammo right now. The gun isn't worth shit if you can't shoot it.