Definitely go with the Ranger/Rogue. Everytime you take a Ranger level, you can throw the skill points into hide & move silently & search, among others. Every time you take a Rogue level, throw the skill points into the other Rogue skills (open locks & disarm device). You could probably get away with a 3/1 or 4/1 Ranger/Rogue ratio and still have an effective Ranger/Rogue.
On the Monk/Cleric multiclass:
1. As stated above, the "Arcane Castine Failure %" only pertains to arcane magics, not divine (i.e. cleric or druid) magic.
2. You might be able to get away with the monk/cleric mix, but the monk WIS AC bonus will be far less than you'd get from wearing good armor. There's simply no benefit for a cleric not wearing armor. You might have some perfectly legit role-playing reasons and that's up to you.
If you really want to play a monk/cleric, you might want to consider taking 2 or 3 levels. I think that at level 2, a monk gets Deflect Arrows feat free. And at level 3, he gets Still Mind (+2 to savings throws against enchanment spells) and increased speed. Something to consider.
Unless this is for roleplaying reasons, I'd strongly suggest going with a single class cleric. Those last few levels will give you some very useful high level cleric spells that IMHO cannot be balanced with a small handful of monk levels (Mass Heal comes to hind
).
Also, since you already have 2 tanks (ie. the Ranger/rogue and your barb), a high DEX cleric sniper sounds pretty good to me.