Most aircraft developers have repaint instructions/disclaimers in their paintkits, or on their websites. AFG is no exception.
Is it Ok to use default parts in your repaint?
Yes, you do not need to repaint every single pixel.
Note: if a model uses many texture sheets, and you only paint one or two, it is common to only upload those that you have painted and instruct the end-user to copy the rest from another livery.
Will textures from an older model work on a newer model?
Maybe, depends on the model.
Do both models use the same filenames?
Is the layout the same between models, on all the texture sheets?
Are both models for the same version of FS?
If the answer is yes to all then you may be OK.
hope this helps.
WWOD---What Would Opa Do?
Farewell, my freind (sp)