Hi tweeny Im no zbrush expert but will try and help
1.I would say 36 was a little high, things like pocket would have been better made in zbrush because as you make creases in the trousers you will have to change subtools to move them.
2.If you import them into zbrush as a pair symetry will still work on them
3.Do all your modelling first in the high levels then drop down to your lowest level then create UVs. you will need to combine(group) you model at some point to get all your UVs onto one map, use subtool master to do that.
Hope this make sence.........dave