Increasing the archers upkeep to 200 would be ok, but then the HP and Peltasts would still need a lowered upkeep, to perhaps 140 and 160 repectively. They are after all only 30 dinarii cheaper per turn. Not much really, and definately not worth it.
Making the archers having the same rating as the slingers would be good I guess, it would also help the slingers a bit as they were supposed to be better than archers in general.

What does the 'highly_trained' line mean? Formation, responsiveness or actually capabilities? Tell tell tell...

Archers could indeed fire in the rain, not much of a problem there. The bowstring would stretch a bit and it would impact on power and range, but accuracy would still be good enough as the archers could compensate for hte rain's impact on the arrow. Wind on hte other hand was far worse for accuracy. It normally strikes in gusts and is thus impossible to counteract or interpret properly.