1. I don't think either of them would engage in a straight fight because what would the point be really? If the T-1000 wanted to kill Odo he'd find a way. If he gets his hand on a phaser Odo's really done.
2. On modern day Earth they would come up even. In Star Trek's future probably Odo. The T-1000 is made of liquid metal
. Surely the advanced sensors of the 24th Century would detect the T-1000 in a nanosecond.
3. They're both made of liquid. I imagine an ice planet or a lava planet would fuck them up pretty royally.