Well met!
I can answer your questions three. I have run a rogue centric campaign and they are a lot of fun! Be ready for the not so normal adventures though. There is a list of questions you have to answer and then make some direction decisions based on your answers. Firstly, do the characters belong to a guild? If the answer is yes, then how the characters get their adventures is fairly obvious. Each job is dictated by the guild handler. If you go this route might I suggest picking up a copy of the AD&D 2E supp "Den of Thieves." Though the mechanics are outdated, the set up of a guild is priceless and invaluable material!
Also, do you envision lots of "classic" dungeons or are your restricting (loosely of course) your vision to mostly urban areas? If the second character decides on a druid, your first character should look to be a bandit and resort to highway robbery. (Literally) Meaning your guild would be a bandit camp. This could be in the vein of "Robin Hood" or something more a kin to every bad guy camp you have every created as a GM. If the second player chooses a more contemporary rogue, I would read "The Fafarhd and Grey Mouser" Books or "Sanctuary - Tales of the Vulgar Unicorn" to get some ideas of stealthy plot lines.
Political intrigue is also a very good plot device for stealth related campaigns so reading dramas and thrillers based on crime and the legal system (ala John Grissom (sp?)) may be worth noting as well. "The West Wing" may also provide some inspiration as well as watching C-Span and the nightly news. A good example is the moment in "Braveheart" where William Wallace fights the lone British Soldier only to find out that it is Robert the Bruce. A great "WHAT!!!??!!" moment, historically inaccurate, but great story, which brings me to point four...
Historical accuracy in this kind of campaign may not be your friend. I am always railing about the historical inaccuracies of campaigns that I play in and run mine fairly close to reality to give it that "gritty" lived-in feel, with this sort of campaign, you may actually want to go as far from reality as possible, because many of your games will be story driven. Actions and adventure yes, but without a cohesive and in depth plot, it quickly becomes, steal gold, kill people, run from law, which gets old and boring after about 6 or 7 games.
And speaking of law or in their case Law - don't forget that your players are playing characters on the "wrong side" of the law, their reality will be skewed from day to day reality. Paladins don't like law breakers and the town guard may have posted notices (Wanted posters) for the player's arrests. Bounty hunters looking to get rich off their pain and the young adventurers' (reverse the normal "go kill the bandits for some cash" scenario and roll up some parties to go after your anti-heroes) will always be a pain in the butts.
To answer your second question - yes full XP - overcoming is overcoming, be it by sword, brain or luck.
And your third - very carefully! Remember that technically you have an underpowered party. (I have the opposite problem I have 11 players) You may have to just play around a bit and be lenient at first in order to figure out what really works. Don't go easy on them, but understand that if they are in over their heads because of YOUR mistake, give them some grace. (Being dumb and doing something stupid of course is fair play and TPKs are always welcome.

) There is a formula somewhere in the DMG, but since I don't have my books at hand at the moment I can't quote it. (someone have that one handy?)
Hopefully you find this helpful, just my two coppers. Happy Gaming!