I started experimenting on the breadboard. I tried different capacitors for the 555-timers and picked the ones that suited my project. When I was testing the synth I didn't use all the potentiometers, LED's and transistors. I only used 2 potentiometers, 2 LED's and 4 NPN transistors because I didn't have enough space so I could only hear two notes, witch makes it harder to choose the right capacitors for the IC's. Why? Well, because I can't tell how fast the loop will run at when I connect all the components so I can hear all the notes one after the other. The combination of the potentiometer and the capacitor affects the speed of the loop. I hope you understand, if not ask a question in the comments!
After I made the Synthezeiser I made th schematic.
I came to the conclusion that a small capacitor should be used for the sound-producing 555-timer and that a bigger electrolytic capacitor is better for the speed controlling 555-timer
Here is the full breadboard test online ( with 8 notes: https://123d.circuits.io/circuits/1933783-8-note-stepsequencer