Is it a good idea before starting out to download all the available extentions or do some clash with others?
I would just download the extensions that you need. Some of them do clash with others (often they’ll have a warning that says which ones they’re not compatible with), but more to the point - a lot of the ones on the Inform website were built for previous versions of Inform and are no longer compatible. The Github page seems to be a little more reliable, but even so - I can’t imagine needing every extension in the listing in a single project.
Extensions don’t clash when they just sit on your hard drive. There’s no harm in downloading whatever extensions you want, but there’s no point in including extensions to your project if you don’t use them.
I’d recommend downloading everything from the Public Library. It makes it a lot easier to read their documentation, and you can experiment with them quickly.
Extensions from the Public Library should all be compatible with the latest version, and won’t conflict with anything if they aren’t included in the project, as Juhana said.