Steam hacking?

Can u have on steam and use Crazy Client?

Yes. Just replace the swf used by steam and it should work.

For me (yours should be similar), the steam RotMG folder is located at:
C:\Program Files (x86)\Steam\steamapps\common\Realm of the Mad God\

So just rename the hacked client you want to use to "AssembleeGameClientProxy.swf" and move it to that folder to replace the existing swf.

Then just run RotMG like you normally do and it should work.