1. for herb/mining. people tend to like pal/dk for mount speed or druid for gathering in flight form + quicker herb time + instant shift in and out of flight. i personally have used druids on all the farmers ive had, but gonna give dk / pal a jam at some point too.
2. you will need to work out whats best for you. IMHO, optimize your farm on a single server before trying to expand. get your shit running nice and smooth. for me, i have 2 farming accts, 1 acct thats multiple 90's for tradeskills (some tradeskills can get away being 85)(the more 90s the better, my plan is to fill the accts with 90s for multiple daily cooldowns(its not much but its still profit)). then i have a 4th battle chest acct that is a AH bot / gold seller that has nothing on it except a level 2 character and if it gets banned from selling gold to asians then i dont care cause i only paid $5 for it.
as for population. no idea. ive only ever played on the one server i bot on, i havent even looked into market prices on other servers. you could use the underminejournal.com to assist in picking a server. my server is full and the second server i expanded to just recently is full-locked and prices seem pretty similar there, so yeah.