So I want mod_rewrite to operate but if there's a folder already existing with the rewrite's name; go to the real folder.

This is the code:

RewriteRule ^([a-z0-9-_]+)/?$ /page.php?category=$1
RewriteRule ^([a-z0-9-_]+)/([0-9]+)/?$ /page.php?category=$1&page=$1

So I am trying to go to the admin folder and it thinks it's rewriting it to page.php?cat... How can I fix this?

Just write another rule over it with the [L] trigger:

RewriteRule ^admin /admin.php [L]
RewriteRule ^([a-z0-9-_]+)/?$ /page.php?category=$1
RewriteRule ^([a-z0-9-_]+)/([0-9]+)/?$ /page.php?category=$1&page=$1

I didn't test it but it should work.

RewriteRule ^admin - [L]

Thanks guys.