the differences are subtle. mới, được, thôi in your first sentence is used to emphasize and show emotion of speaker. the structure N +mới +V +được +( một ít, một chút, một tí) + O + thôi is used to show your dissatisfaction of the result of the job that you've done. for example, in your first sentence, the speaker want to imform the listener of his dissatisfaction and disappointment and somthing happen to prevent him from countinuing although he want to eat more. and another example, you're having a test but time's up, you have to hand in, you complain: tôi mới làm được 1 tí thôi. Tôi ăn một chút thức ăn speaker don't show emotion just want to tell you what had happend for example, tôi ăn một chút thức ăn rồi đi ngủ,the sentence is often used to tell a story,anecdote, or account of a event. Tôi ăn được một chút thức ăn thôi is pretty much similar to your first sentence,but in some case not interchangable. Your first sentence shows more intense feeling and is used to complain bitterly, especially emphasize on something prevent you from countinuing and you hate it, you don't want it exist.Tôi ăn được một chút thức ăn thôi just show dissatifaction. i hope it's helpful.