Here are the mistakes in each sentence along with the corrections:

5. **Be less careful when you eat some kinds of food, my dear! You may have allergies if you don't take my advice.**
- Mistake: "some kinds" should be corrected to **"some type"** (it is more common to use "type" in this context).

6. **She likes eating fish, so she also likes drinking mineral water.**
- Mistake: "so" should be corrected to **"and"** (to better connect two similar ideas).

7. **It has been two weeks since he has worked for this non-profit organisation.**
- Mistake: "has worked" should be corrected to **"worked"** (since "has been" is already indicating the present perfect aspect, the past simple "worked" is more appropriate here).

Revised sentences:

5. Be less careful when you eat **some type** of food, my dear! You may have allergies if you don't take **my** advice.
6. She likes eating fish, **and** she also likes drinking mineral water.
7. It has been two weeks since **he worked** for this non-profit organisation.