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Edxactly, Welcome to the XOOPS forum!
1 OK There are more then 1000 themes available.
2 XOOPS permission system turns around user groups. You could make for every user a group and then in some calendar modules a category for each group (=1 user), but that will turn out too complex after a time.
I'm wondering by the idea that a user should visit a site to know when his pool is sheduled. I think sending emails with their sheduled date would be more friendly and practical or useful.
Maybe a module as clientspace or xHelp could provide in some space or message board for doing this.
3 That is possible, but you must take in account that for making the payment, the same data will be asked as for registering and that when registered some of the payment data can be remebered and thus avoid entering them again.
I think you need a shop module like Oledrion to sell one product: the pool service. Maybe there are variations and you can have more then one product, like deluxe cleaning, or sell maintenance or other pool products too.
4 5 6 NO, This is typical something you do with an accounting, invoicing and bookkeeping program. Makes not much sense to have this on a website.
7 OK Module like News, (see here on this site) allows to write articles with online WYSIWYG editor.
8 OK Forum, guestbook, polls, formular modules are available.