According to the Lewmar Delta chart, 25 kg should be plenty for your 42'.
I think the Delta is not a bad anchor but, nonetheless, I prefer a new generation anchor such as Spade or Rocna as I believe they are more likely to re-set after a wind-shift in the middle of the night.
The windlass should have no issue with the weight difference of the different anchors. The bulk of the weight it lifts is the chain, not the anchor.
I have a 40 kg Rocna in my GB50 and it fits well in the bow roller, which is similar to the one in the GB42 or, at least, to the the older 42s.