I like where this is going

Good start.
Starcraft is best for story due to Stargate more advanced tech and time travel making no sense or logic, C&C has absurd energy tech with absurd teleportation.

PS If you want to make good cross make it so that "God" who teleport them is Precursor and he is making it just to test how much difference they will make. (Precursor are more energy being, and they let Proms to "kill" their body)

And dont touch absurd tech, because it makes even good written stories a bit stupid...

If the crossover with stargate is also better to strengthen the earth's forces to face enemy forces

Bro, what if the story crosses over with the Gundam Seed before the bloody Valentine tragedy

isn't that punic-class supercarrier actually using 2 "super" mac in canon ?

Awesome intro.
Looking forward to more.

As for the world selection, I would say I am torn between SG and SC but it depends on what timeline you will use.
C&C I am not interested, unless it was Tiberium or Red Alert.

As for the OC slots, could you give us a blank template.