There is no real convention, no.

My rule of thumb is that if it makes the code cleaner and easier to read and maintain (this last point being the most important), then use streams. If not, then use normal loop iteration.

not for this task, way too slow

No, it will fail with an ArrayIndexOutOfBoundsException.

I did not know about the removeIf method until seeing your solution, very useful!

The power of ES6

Kata says:

Threshold K will always be reachable.

Cool

Wow. It's cool

Nothing. Regular string concatenation faster that String#format. You can find in google performance of regular concatenation and String#format.

In case when sum of all numbers < k your code will be execute infinity

If the passed array has no missed letters, your solution will end with ArrayIndexOutOfBoundsException.
For instance, ['a', 'b', 'c', 'd', 'e'] -> ' '.

bad performance