2D Unity project. Floating Balloons

Write a 2D Unity application. Here are the specifications:
1. There are balloons on the screen
2. The balloons always fly to the top of the screen like balloons filled with helium
3. Bigger balloons fly higher than smaller ones
4. The balloons must look as real but slightly cartoonish
5. The balloons can't fly higher than the top of the screen
6. The balloons fly as real balloons so they can collide and must change their direction according to the laws of physics
7. User can pop a balloon. When user pops a balloon other balloons floating around it must fly off a little
8. User can change the size of a balloon. An increased balloon must fly up. A decreased ballon must fly down.
9. User can add a balloon
10. User can write something on a balloon
11. All changes on a balloon must reflect on other close floating balloons according to the laws of physics (when you increase other ballons must fly in other direction. when they collide they must change their form according to the collide)
12. Balloons can't overlap each other. So it's a 2D application. But balloons look a little 3D

The best video on youtube which I found looks like https://www.youtube.com/watch?v=SyZa7yqMJMg For me it's the best representation what I want in terms of graphics.

So saying 2D unity application I mean
1. Source code
2. Unit tests
3. Sprites (I guess the main difficulty here. They must be made from scratch so I can use them without breaking laws)