1.) Yes. You can pre-select the "results per page" setting by either linking to the search page with an extra parameter (eg. instead of simply linking to "search.php", link to "search.php?zoom_per_page=20") or you can define your own search form (see below) with the per page option specified in a hidden input tag.
2.) Yes. You will mostly likely want to disable the auto generated search form, and define your own search form as described in this FAQ:
You can then specify the available "results per page" options as you would like. If you wish to force it to a specific number, and not allow the user to change it, you can use a hidden input tag like so:
By having this in your search form HTML (and omitting the dropdown box in the example from the FAQ), you will force the search to 5 results per page.
<input type="hidden" name="zoom_per_page" value="5">