Sign-Up questions with BuddyPress installed

Hello,
I have BuddyPress installed and network wide activated on the main site.
1. username has to be in lower case, but how can I inform people about this? In my test, if a user enter username with a Big letter, submission will fail. Is there a way to provide hints to make the registration easier and more smoothly for new users?
2. I have a piece of code which make the new blog option checked as default, with the help of you guys. It works fine. But I notice one thing, if users failed to submit the registration due to capital letter in username, for example, the new blog option will become UNCHECKED. How to make the new blog option always CHECKED? Here is the code I have.

<?php

add_action( 'wp_head', function() {
    ?>
    <script type="text/javascript">
    jQuery(function($){
        $('#signup_with_blog').click();

        $('#signup_username').keyup(function() {
            var val = $(this).val();
            $('#signup_blog_url').val(val);
        });
    });
    </script>
    <?php
} );

3. In the BuddyPress registeration form, I am able to modify the BuddyPress part, but I don't know how to modify the WordPress part? How to do that? I want to add some hints to guide user to complete the registration smoothly. For example. the username has to be in lowercase and has to be letters and(or) numbers, pure numbers is not allowed, and has to be 4 or more characters. However, people are not aware of these rules and very likely will cause problems and unhappiness in the registration phase. I really want to improve this.

Thanks for your help!
YZ