Learn Portuguese (Portugal): Ele estudou pouco, portanto reprovou.

In Portuguese, you can drop the subject pronoun when it is already clear from the context. Since ele (he) was just used as the subject of estudou, it automatically carries over to the second action, reprovou. You don't need to repeat it!

I know um pouco means 'a little'. Why is it just pouco here?

There is a big difference between um pouco and pouco. Um pouco means 'a little' in a positive way (meaning he actually did some studying). Just pouco means 'little' or 'not much' in a negative way (a lack of something). Since he failed, it makes sense that he studied pouco (not enough).

Can I use por isso instead of portanto to mean 'therefore'?

Yes, absolutely! Both portanto (therefore / so) and por isso (for that reason / so) are excellent ways to show cause and effect. Por isso is extremely common in everyday spoken Portuguese, while portanto is just slightly more formal but still widely used.

Does reprovar mean to fail at anything, like a general task or a job?