Are these sentences OK?
I just finished eating.
I already finished eating.
Once you get finished eating, come over here.
After I've finished eating, I'll come over.
Can't you just wait until I've finished eating?
Can't you just wait until I'm finished eating?
Have you finished eating your lunch / dinner?

Question

Are these sentences OK?
I just finished eating.
  I already finished eating.
  Once you get finished eating, come over here.
  After I've finished eating, I'll come over.
  Can't you just wait until I've finished eating?
  Can't you just wait until I'm finished eating?
  Have you finished eating your lunch / dinner?

Answer

I just finished eating. - should be Present Perfect here (but in AmE Past Simple is also possible)
I already finished eating. - same as for the 1st sentence
Once you finish eating / or Once you've finished, come over here.
After I've finished eating, I'll come over. - You can also say, "When I finish.."
Can't you just wait until I finish eating? - both sentences (including the next one) are correct
Can't you just wait until I've finished eating? - 
Have you finished eating your lunch / dinner?

Ne manquez pas cette occasion d'apprendre une langue depuis chez vous. Découvrez notre sélection de professeurs de langues expérimentés et inscrivez-vous dès maintenant à votre premier cours !