3
Your question is not stupid
I know a few people who run three or more XOOPS websites from the same db. I know one guy who has 8 websites (not all xoops) and a few separate forums, inc phpbb, etc. on the one database.
To run several XOOPS websites from the same database is perfectly possible. Best way to do it is to do a separate XOOPS install for each website but choose a different database prefix for each site. This will make the sites independent.
You can use different themes/styles for each website, no problem at all.
Thew only thing I'm not sure about is each website using the same Forums - I think that this will involve a lot of hacking and code changes, but I could be wrong. I'm sure that an expert will be along to contradict me any minute!! Likewise, I'm not sure about one registration covering all sites.
The one thing that you will have to look out for is that your server and database will have to be up to the task if any of the sites are well used.
HTH