Lets say you do a run and kill 25 titans, could you submit the same run to the 10ks with it cut accordingly and also to the 25ks? Or would you need to just choose one to submit it to?

You can submit to multiple categories with the same recording, just remember the run must start at a valid spawn point.

So if you were doing 50 kills and got an extremely fast 10ks in the middle somewhere you couldn't trim that out and submit it as you need to start at spawn with 0 kills.

But in your example, it would be fine with me.