Should I try re-screwing those sides?
If the core is misthreaded re-screwing wont help. You would need a new core.
What do you guys recommend for switching to CN? I am currently practising, but don't know how to practice. Should I:
1. Solve with a different colour cross everytime
2. Solve with the same colour cross for a day or so and switch to another for the next day
How fast are you now? CN only helps to find an easier cross case whilst slowing F2L down for quite a long while until you get used to it. If you started cubing CN then sure, it would be ok. However learning CN is useless in my opinion. Unless you're like sub 10 and still not CN.
Read this:
The most often chosen side to make a cross on, by convention, is white. Most people are thus said to make a 'white cross'.
However, there are a few people that are called color-neutral. This means that they first decide which cross looks easy to make, and then they make that cross. This approach saves about 1 move in the cross stage, but it is often argued that it is not worth doing it because of slower recognition times especially in F2L stage, because the color scheme is ever-changing.
There is yet one more class of people that make dual-color crosses. They choose to do either white or yellow cross. This decreases the number of moves in cross stage, but arguably doesn't change the F2L recognition times by as much.
This issue often leads to a lot of discussion, and there is no definite consensus on what cross color(s) are best to use.
Lars Vandenbergh analyzed this issue quantitavily and the whole page can be seen
here. Here are the results summarized:
Averge number of moves to solve white cross: 5.81
Averge number of moves to solve dual cross: 5.39
Averge number of moves to solve neutral cross: 4.81
Looking at the results I am personally convinced that it is best to choose one color and stick with it
