WP multi Site network and domain mapping

Hi everyone, I am a fairly new webmaster and fairly new to SEO so any help is much appreciated.

I am wondering if I set up a site for my business like mybusiness.com and then I want to have a site for each of the brand I sell, should they be on their own hosting account or can I do a wp multi site and have domains mapped to them

I was thinking I could do brand1.mybusiness.com and have brand1.com mapped to brand1.mybusiness.com would this increase the SEO of mybusiness.com or hurt it? It would be nice to save the money from having to have 2 hosting accounts as well as being able to manage them both at the same time but I would rather do what would be best for SEO.

I am under the understanding that if I did mybusiness.com/brand1 I could have the brand1.com domain direct to it but it could cause problems with duplicate content? Also, I could not install a custom WP theme on that folder correct?

If someone could explaint his to me that would be great