Hi Dave,
Office 365 for Home Premium Preview is a product of Office family, it contains powerful new versions of Word, Excel, PowerPoint and some other powerful applications and services. However, it doesn’t provide the online service (e.g. Exchange Online, SharePoint Online) which provided in Office 365 P plan as you use.
If you want to use both Office 365 P plan and Office 365 Home Preview, you should purchase for both. However, currently, as far as I know, Office 365 Home Preview is free and you don’t need to purchase it.
Thanks,
Eric Sun