The 18w lamps should work OK with the 20w ballasts you have and the plants will adapt to whatever light you put over them, so no worries on that score.
However, I did see a reply from Clive recently about changing lamps and he advocates that there are only 2 reasons that you have to change a lamp.
1. If it don't work when you switch it on.
2. If you fancy looking at a different colour.
The plants don't care either way, just don't get caught up in the manufacturers hype, that's all
Yes they will.
The Skywhite is a bright white lamp and the Grolux has a pinky tinge to it. The combination of the two reduces the harshness of the overall lighting effect. Looks pretty good.
I'm using something similar myself .