Custom blog registration

Hi, It´s mi first post. Sorry, but I don´t speak english very well, I´m from Spain.
I´m here because I need some WPMU DEV plugins, like Easy Blogging, Set Password, and some more... and also the support forum, where I learned a lot reading, but had not written anything yet. But now I have a problem that can not find any solution or plugin to do it.

I´m seting up a new WP Multi-Site in Spanish language, with subdomains, for create free blogs to the users, and to put advertising inside all blogs, sharing advertising with the users, to promote more users registration. The site is: centraldeblogs.com

I´ll put 1 top banner modifying the themes with an <?php include("ads/728x90.php"); ?>, and the ads script in an external php archive (I have not found any plugin that do this for Muti site, only WPAds for single user) , and the users can put their ads inside their posts througt Simple Ads plugin.

I tried to make the use of the blog very easy for my users, so I'm using Easy Blogging.
But the first step for my users is the registration, and is difficult to the begginers.

It´s the problem:

Basically the problem is that Wordpress is preparated to create users, and the users can create blogs. But I only want create blogs with an username and pasword to login it.
The reason is that: in wp-signup, the form ask for an username, but in any moment, the user knows that this username will be their blog addres. (http:// username .centraldeblogs.com)
And below, the radio button asks for “Create a site”, or “Create user”, but in any moment appears “Create blog”, which is what my users are looking for.
Seems silly, but the 5 blogs that I have registered, they are all my friends, and 3 had to ask me how to do it, because the buttons were not clear.
I want it to be very easy, so that even my mother can know how to do it. :wink:

I have enough time searching for information online, but I have not found a plugin that does this, and I have no much knowledge of programming to do it myself.

In short, I need this on the registration page:
1.-the form of username appearing preceded by "http://" and followed by ".centraldeblogs.com" to let the user know that he is creating an web address.
2.- to check the username availability, like here
3.-radio button disappear (creating internally the user and the blog).
4.-and also show the form for the blog description.

Can anyone help me please?
It is essential to attract new users, but the process seems complicated for beginners.

Sorry, it´s very long...