I ate so much food so late at night.
I ate plenty of food so late last night.
Are these sentences okay?
They're grammatical, but I'm not sure you'd ever hear them. On the other hand, for a picturesque phrase, you might combine them and say:
1. I ate so much food so late last night.
An alternative, non-pithy version would be:
2. I had too much to eat before going to bed last night.
Not a professional ESL teacher.