Are flat AP, and AD runes a good idea to use?

For what kind of runes?

I mean, in general, Flat ad is better on ADC's, and Flat AP is better on early game mages. You can run scaling ap for glyphs instead of mr or cdr.

It really depends on the champ you play. On champs like Mordekaiser or Ryze, I take MS quints because of their lack of mobility, especially Morde. If you are playing someone like Kassadin or LeBlanc, you probably want AP quints. Any ADC would use AD quints.