there's a fixed encounter when you first enter the upper monastary with 2 rabid rats, and there's another, with 6 gnawer friends, in the anti-magic room. so it's not entirely a matter of level that you'll run into those badboys.
i've only had one party get nipped for a disease tho. not only did we have no alchemist for mixing, but we had noone with healing or identify. they were restarted with a slightly better party chemistry
as a big bard fan, i'd strongly recommend following the *traditional* plot line to get the best instruments in the timeliest fashion. that is:
s
p
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s
p
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monastary (sleep, bless)...
road (shrill sound)...
arnika (charm)...
road (cemetary for insanity)...
trynton (slow)...
swamp (log bridge for haste)...
mine tunnels (silence x 2)...
trang (heal all, magic screen, armormelt)...
teleport to arnika hut (freeze all)
the key is the four items at the trang.
the alternative is to try to get to the trang by way of the umpani, which might get you there earlier. but you have to fight or pickpocket kunar to get there (to use his teleporter). this locks you out of his trading and a quest later. you may get some of those items before you can use them, which makes it seem something of a waste - especially since you miss out on the haste drum and insanity horn.
i should mention that going by way of the umpani gets the freeze flesh and soul sax (if you know how). but this doenst really make it a better choice. the freeze flesh is obsolete once you get freeze all and soul sax isnt really needed until much later in the game.
note, however, that i'm evaluating the plotline choices strictly on the basis of bard instruments. there a lots of reasons to go a different route!