What are the rules for Storm Giants?
According to the first wiki article you linked to, Storm Giants are most likely a successor chapter of the Salamanders, which means they should probably be played using Salamanders chapter tactics.  However, also according to the same wiki article you linked to, (in universe) the true founding chapter of the Storm Giants is unknown... The fact that it is the Salamanders is just the Imperium's best guess.  So really, the Strom Giants could be a successor of any chapter, and therefore you could use any chapter tactics you desire without breaking canon.

As Aslum pointed out, the references section of the first wiki page you linked to will give you more information. 

EDIT: Aslum deleted his post... However, his advice about checking out the books listed in the references section of the Storm Giants wiki page is spot on. (should you want a physical book reference)