Well i don't use BTYahoo but i am guessing you should be able to use both.
Now for the second question's answer... Go to outlook express then go to tools, accounts, press mail tab, press your account and press properties. Then press the connection tab and if the tick box is ticked, untick it. Try again. If it comes up asking for your account name and password and does not work then try different characters (e.g you usernames first letter lowercase instead of uppercase). Hope this helps.