Outlook 2007

I manage 8 email accounts, some are gmail (imap or pop3) and others are only
pop3 accessible. My main (default) email is my gmail account.

I've configured gmail in outlook via IMAP and it's working perfectly,
however Outlook has created a separate set of mailboxes for this account
(separate from those in my 'Personal Folders'. I would like all accounts to
have their received messages in one single inbox, but when I REPLY to those
messages, the 'from' address should be from the account that the message was
originally delivered to. How can I do this?

To get 1 step more complicated, some of the other email accounts are also
gmail and they are currently set up as pop3 but I would prefer to use imap.

Thanks so much for any assistance!