I have now managed to integrate TinyMCE, FCKEditor and Xinha to moodle together with HTMLArea by taking away functions print_textarea and use_html_editor from weblib.php, pasted those functions to new modified files for different editors (weblib1.php for htmlarea and all normal themes, weblib2.php for tinymce and so on), placed init code of editors to theme header.html and meta.php (to get different editor configurations and skins for different themes, configurable themes have meta included true and standard meta included false in theme config.php), created a editor (theme) selection block for teachers and admins ("raw version") that uses right now session themes to render any editor and I am now testing different configurations, plugins, loading times etc with different browsers. All of these four editors have some features that make them better or worse than others, for example Tinymce works with Safari without any major problems but has some conflicts with the latest overlib.js in IE (old overlib worked ok)...
Well, selection system is working and upgrading moodle or editors any time is pretty easy but I would like to get some advice from "senior developers" - what is the best way to replace old editor or add new ones? I guess that editorlib.php was originally made for this purpose...and where to put that "editor selection drop down list" (admin panel/block/user fields/over editor textarea...)?
I will give a link to my test site soon with my test files for moodle 1.8 and moodle 1.9. I must still finish some buttons and fixes for IE6 and IE7.