Yes you can sell your personal products to your customer and bill them through your company GSTIN. Just note that the company will not be able to claim ITC since company's GSTIN was not mentioned in the Purchase Invoice.
Coming to your second question - GST will be levied at the applicable rate of Goods as prescribed under Notification No. 1/2017 - Central Tax ( Rate ), dated 28th June 2017. So if your goods attract 18% GST, then charge 18% GST only from your customer irrespective of the fact that the BCD + IGST was 28% while you imported the goods.
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