No you cannot, Wow will only allow you to log onto 1 character per account , at a time.
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If you mean merging 2 wow accounts so all the characters are available from 1 account, that is not possible. It is, however, possible to merge up to 8 WoW accounts under a single Battle.net account. You can do this by going into your Battle.net account and adding your second WoW account to it. This can only be done if both accounts are registered under the same name. You will still have to log into an account separately to access different characters, but you can start up WoW twice, and log in to a different account with each session. Addendum:The above answer isn't correct. You can certainly merge 2 accounts as long as your name is registered to both. The only exception to this I believe is if one of the accounts belongs to a minor over whom you have guardianship.
Yes, you can use any addons that a full account can use. There is no limit.
A trial account itself is not the issue, it's the installed program (client). Once you have that installed, and you have made an account with a private server, it will work regardless of you also having a trial account at Battle.net.
When you install WoW, it install all expansions and patches up to the current client. This is even true if you do not upgrade your account to BC or Wrath. So it will take about 15 gigabytes.
No, the WoW free trial only includes the original game. There are no expansions in the free trial. Also while playing a free trial account you are not able to receive mail, whisper other players (unless they have you as one of their friends), or talk in channels (i.e. Trade, General)