FCKEditor can be a pain in the butt when it comes to it's use of the <p> tag. You can see this post for a rather lengthy discussion about it: http://www.mojoportal.com/Forums/Thread.aspx?pageid=5&mid=34&ItemID=9&thread=3463.
The next version of mojo will have an upgraded and enhanced version of TinyMCE which will be the default mojoPortal editor for new installations. TinyMCE doesn't have the issue with the <p> tag like FCKEditor does and Joe Audette has added a lot of functionality to TinyMCE to show thumbnails of images when browsing the file system. The Content Templates and Styles are also available in TinyMCE. I prefer the way TinyMCE shows the content templates in it's template browser; it displays an actual preview instead of you having to create a thumbnail screenshot of it and upload it to your server.
You may want to login to the demo site at http://demo.mojoportal.com and experiment with TinyMCE.
The short answer: wait until the next version of mojoPortal and use TinyMCE. The last I heard on the next mojo release was that it would be out some time today; Monday, August 24. Of course, that may have changed.