A problem I used to get with the twin carb set up giving erratic idle settings was that the carbs were not mounted correctly, by this I mean the spindles were not truly aligned. The mounting studs allow enough play for this to happen effectively allowing the spindles to bind against each other therefore not always returning to the same rest position. This was never as obvious on used carbs as there was always a bit of slop in the bushing but as you say your carbs are new and therefore a bit more unforgiving making the alignment issue more important. Try slackening off the flange mounting nuts and realigning the carbs.
As regards mixture, plug colour is the best way to determine same, start richer rather than weak and tune in. I presume the needles fitted are suitable for your set up, although at idle most of the needles are the same.