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In short....yes. You can do all of that. The XOOPS Groups admin is flexible and powerful. You can create an unlimited number of groups, and assign specific permissions to all of them.
Adding static HTML is possible via 3rd party Modules (like Tiny Content) as well, but static pages don't register in the database, so you lose many of the features provided by using dynamic content. For example, it will not show up in search results.
If users will contribute articles, they could use an article module like WfSections, or AMS to subit said article, using HTML (if you allow them), or XOOPS code (just like BBcode) to format, hyperlink, and insert images. That way, you can still have all article register with the database, yet still be very simple to write.
You can even use one of the menu modules (like Content) to add links to specific articles or pages within xoops, or anywhere on the net really, within the main menu, or you can even add additional menu blocks with the article link.
Really, you can do ANYTHING with XOOPS you need. But in some cases, (usually in depth, very unusual stuff) it may take some hacking. The whole core is very intuitive, so if you take the time to REALLY learn Xoops, the sky is the limit.
And lastly, you will find the XOOPS community to be one of the largest, most helpful and friendly communities out there. If you ever have a question/issue, come to the XOOPS site here, and generally, you will find your issue/problem has already been addressed, and if not, a simple post here will help you figure it out 99% of the time from my experience.
Hope you get started using XOOPS soon, it truly is the best CMS out there, hands down.