Yes you can mix spring rates.You will come up with odd rates,ex. 43 and 42 would be 42.5 overall 43 and 45 would be 44.To figure this out add the two rates together and divide by 2,that will give you the spring rate that you would have.
it dont hurt till the bone shows.
KTM's came with White power (I believe) forks a while back that had compression damping on one side and rebound damping on the other. So I doubt springs of differnt rates is a problem at all. I do it frequently when testing.
One thing to consider is that there is such a thing as manufacturing tolerances. They are 5% for most OEM's. So a .42kg might not be a .42 by direct test. If you want to be sure you have to test each spring.
After a 70 mile Enduro yesterday I can attest that what I felt was better compliance on the small stuff, with a slight loss of whoop stability (wants to hobby horse).
I put one .40 in with the stock .43 for an average rate of .415. Much easier to pick lines into turns. If there is twisting, unequal flex, etc. I can't feel it.
Any suggestions to stop the hobby horsing? Guess I need to recheck the sag (may be high as a consequence of lowering the front with the softer spring rate) and maybe slow the rear rebound a tad as it feels too active. At a reset I tried more front compression but then my wrists took a pounding and it resisted turning in as well. Didn't have time to soften the rear compression, but would I soften hi or low to level out the bike? Too many questions.