An update.
I solved on my own the problem related to the multi-session with MSN 7.0.0820 thanks to an absorbation's post from here
http://shoutbox.menthix.net/showthread.php?tid=76600 , which said to install the StuffPlug-NG plug-in for Messenger Plus 3.63.148.
I also found from the link given here
http://www.msgstuff.com/downloads/file/23/ that the website contains other plug-ins like Color Nick v2
http://www.msgstuff.com/downloads/file/25/ ,which I hope will allows me to show properly the contacts' nicknames with colour and other tags, since the current Msn Plus 3.63.148 seems that it doesn't work, although there's an option to show the colours of nicknames.
I'm still searching for a plug-in to set a password to access the chat logs.
Any help from anyone will be highly appreciated. Thanks to everyone.