1. i is 2.0L.. s is 2.3/2.5L
GX, GS, GT are trim levels (better lighting, heated seats, etc)
2. Go to any Mazda dealer
3. There is a sticky somewhere for tire fitting. If its OEM option, then you can use it.
http://www.tacomaworld.com/forum/tir...5r16-195-65r15
Personally, unless you're in heavy snow area, I would get good (albeit expensive) all seasons.
For winter, General Altimax, Michelin X-Ice
All season, Pirelli P7, Continental ProContact, PureContact. ExtremeContact DWS, Michelin Primacy MXM