I have just finished the game on a normal playthough and obtained all the modokons and i can say that the achievements for hard, normal and easy are stackable :) if only i knew this before playing the game on normal (i could have just done a hard playthrough)
if you find that you finish the game and realise you are missing some modokons (like i was). Go to play game and then chapter select. you will see which levels are not 100%. *note* you must finish the chapter for the modokons to count. i tried saving them and then quitting out by quick saving the game and this didnt work. :( (once all modokons are accounted for you will then need to complete the last level - once this is done the achievements will pop :) )
i used this guide below - this is not my own work - credit to IGN. great guide has a video for each chapter and also descriptive text below incase you didnt want to watch the videos ;)
http://uk.ign.com/wikis/oddworld-new-n-tasty/Walkthrough