Will this work
 

I have a Ampeg 500 watt head with a SWR410. Can i add a Hartke 2200 pro 212 thats 300 watts. Will it fly?

Your amp will probably handle a load of 8 or 4 ohms. I believe the SWR is an 8 ohm cab and the Hartke is 4 ohms. This will give you a load of 2.67 ohms and is not a good match.

and if you post this question in the right place ("Amps") you'll get plenty of responses explaining why mixing different kinds of cabs is not so awesome anyway.

It's highly unlikely that you need more rig. Many 2x10 are more than enough for most gigs.