How to set the default wysiwyg editor (Epoz, Kupu or FCK Editor) for all users in Plone

For all new users you can set it in portal_memberdata->properties
for all existing users you have to run a script that sets this property (*I think*)

In addition to that you can always customize the wysiwyg_support script in
plone_wysiwyg to take precendence for your preferred editor if the user in
question has the default setting.

That is unlikely to have any effect.

Kupu and Epoz define their own wysiwyg_support template which overrides the
one in plone_wysiwyg. There are instructions on Plone.org for defining a
suitable template for FCK Editor, although if you are using Kupu 1.3.x the
support for FCKEditor is already included in Kupu's template so you can
ignore the Plone howto.

If you want to customise wysiwyg_support you will need to find the
appropriate version and customise it.