Is it Safe to Use 2010 MacBook Magsafe with new MacBook Pro 13" with Retina?

Yes, with a MagSafe 2 adaptor.

 

All Magsafe chargers can be safely used with any Magsafe device. The only difference between Magsafe 1 and 2 is the plug, so just use an adapter.

 

Apple sells a MagSafe 1 to 2 adapter for $10 - if it wasn't safe I'm sure they wouldn't allow you to do it.

 

You'll need the adapter, but yeah, perfectly safe. All notebooks work with each other's MagSafe.

The only thing is that using a lower wattage magsafe will cause slower charging and perhaps even battery drain when the computer is at full power draw.

Using a higher wattage magsafe will have no effect whatsoever as the computer is the one in control of how much power to draw.

Since both computers are 13" in your case, the last two paragraphs do not apply.

 

Yes it's safe, but you might need the $10 converter that allows you to use MagSafe 1 chargers with MagSafe 2 ports - the new port is smaller than the old one but they are electrically compatible.