Delete page after redirect?

Question

Search Engine Optimization

There is an old site and a new one, from the old site I published material on the new one, and in the old site in htaccess I registered a 301 redirect from a specific page to a specific page of the new one. Indexing has passed, the pages appeared in the search with new URLs.
1. Is it possible to delete a page from the old site, from which there is a redirect to the page of the new one, or wait? How to know when?
2. Is there a difference when deleting before and after indexing a new page?
3. Or in general, after deletion, the 301 redirect does not have time to work out and a 400 error is returned to the user.
If there are no incoming links to the old page.

Answer 1

1) if there is no old one in the index - you can
(search by the phrase with quotes)
2) well, yes, before - it will throw it out a little longer, like a 404 site
3) see
p2 add to GWT, there you can immediately see a decrease in the intensity of the scan, if 404 went