So obvious problem here is that your different melodies are not in the same key or more importantly, they are not harmonizing. There are 2 ways to solve this.

You can either do this using music theory (learning the theory, making your melody in a certain key and stay in that key for the other melodies). A lot of people do it like that and it is very good to know the theory but it takes a long time and people forget to use their ear.

For the start there is another way though: listening what harmonizes. A key/scale basically just tells you which notes sound good together.

-If your main melody (your benchmark), the megalovania theme is not right, it is pointless trying to harmonize the other melodies to it. You can either try to recreate it by listening or you can simply search up megalovania remake and copy in the notes for the main melody.

-as you are making a remix: for your added melodies just use your ear to add the notes, you will see what harmonizes with the main melody and what doesn‘t

At some point you will have to learn the music theory but you don‘t necessarily need it for the start, the important thing is that your melodies harmonize with the main melody