Are you guys referring to topspin serves and kick serves as separate types?
I think learning a consistent topspin serve to use as your 2nd serve is important as it reduces your double faults dramatically and gives you a safe and consistent way of starting points. You can even use a faster version of the same serve where you hit through the ball more as your first serve. A slice serve is easy to incorporate once you've master this.
Leave the super-fast flat first serves for the practice sessions, not matchplay.
Disclaimer: I'm NOT a coach...
Real tennis: https://www.youtube.com/watch?v=yDqnkLJ9BtM