Pwrsonally I don't see any other coins that are worth daily trading besides Bitcoin. Also, day trading is very demanding, it requests a lot of experiences and knowledge and your full attention to the market so it would be very hard to follow this for multiple coins. But maybe there are some successful traders who could manage that also.
If you are day trading then it is a good idea to try to monitor more coins than just bitcoin since the more markets you watch the more opportunities you are going to have to make money, but you are right, the more markets you are watching the harder day trading becomes, but you do not need to watch hundreds of coins at the same time, as long as you can watch 3 or 4 coins at the same time then you will have plenty of opportunities to multiply your capital.