Newborn Guppys

Wow! My female gave birth to maybe 20 babies, just two seconds after I saw one pop out of its mum,my betta swam up and ate it! He's now in timeout (breeding box) until the babies have grown bigger, Ever since he's stopped continuously hunting for them, them have confidently explored the tank, the neons and catfish leave them alone, so does the male guppy. The females on the other hand have eaten a few so far, luckily they have lots of hiding spots!

only 20? the reality is that in time she could be dropping between 50 and a 100 each time ... sometimes more. I had one female guppy that dropped 170 in one go. I know there were that many because I counted each one into a bag to take to my lfs. They tend to drop more as they mature.
 
All of your fish will eat the young - including the mum. I had one female platy that used to drop a fry, spin around and eat it before it had even uncurled itself.
 
It would be better to catch the fry and put them in the breeder box rather than your betta. Being in the breeder box will be stressful for him/her. Fish have no concept of 'time out'. Their brains are just not developed enough to understand this. If you want to save some of the guppy fry it would be better to move mum when she's heavily pregnant into a seperate tank, allow her to give birth, and then return her back to your main tank leaving you with a second tank to raise the young in

The question is do you want to save them. If you leave your guppies to breed without the likes of your betta eating the fry you will soon end up with a tank full of guppies, very of few of which will bear any resemblance to the spectacular males you bought. If you reckon you can find homes for dozens of guppy fry then by all means try to save them. I think its unlikely though. Maybe best to let the likes of your betta thin them out.