1. Suppose a users registers on uCoz(by going on my website and creating a uCoz account to login) with his info(phone, city, name etc), can I use the info of the registered users when he is posting an ad so that he isn't required to fill up the fields?
Eg- If user A registers with his info-
now when he posts an ad on my site, he can see some of the fields filled in because he is registered.
2. A guest who wants to register on my site, can he do so if I dont have the premium package or he can register to uCoz and login with his uid, bypassing my site registration?
If you have not paid for 'Local Users' your site's default registration/login authorisation is through uID - meaning your registration process is exactly the same as the one uID has in place. As to logging in, any user with a uID account can login to your site, unless of course you have 'Local Users' enabled.
3. "Block activity of users with unconfirmed e-mails:"- Will this block the activity of" Guest" too?
Guests cannot 'confirm' there emails as such so there activity is not bound by this control.
4. What all things lead to decrease in disk space?
I don't understand your question I'm sorry. Could you elaborate further on what you are asking.
5. Where all can i see the group of the user?
If you are meaning on site - you can view users' groups on their profile pages, site users page and forum threads. Alternatively you can search for specific users in the 'Users List' in the Control Panel or otherwise browse through the same list.
6. When i click on an ad, i can see "Added by: xyz"......i want to edit it so it looks like "Added by: xyz(group name)" and can i change the font size and style of the above?
Open Control Panel > Design Customisation > Ad Board > Page with entry full text. From here search for the text you'd like to change and you should be able to do what you're after.