If you want to get a Steelix you must give it a item, called a Metal Coat to a Onix, and Then trade it to a friend, and it will evolve to Steelix.
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You get it on Route 27 inside the cave
use fire types for her magnemites then use a ground type for steelix
Yes, you can. If the Onix is holding a metal coat, it will not prevent you from trading it, and the recipient will receive a Steelix (because Onix being traded with a metal coat will make it evolve into a Steelix).
In my opinion, you have a great team that's very balanced and will do amazing things in Pokemon SoulSilver.
The only way you can get one in Heartgold is Jasmine will trade you one with her Steelix in the National Park (she'll offer it after you beat her for the second time)