MyContacts is a convenience app for collecting my most frequently used contacts in one easy to use widget.
iirc, it depends on those contacts being present in the phone contacts list. I don't think it has the capability to add contacts.
Again - its not a replacement for the n900 contacts app - Its a nicer view of those same contacts.

For Skype: I have a Skype subscription and a Skype In number. I have added that Skype In number to GV. So for me, qgvdial shows me my Skype number as callback option _and_ also shows me spirit:undefined as a callout option. (The undefined part is because I haven't found a way to get the skype-in number for that skype account.)

After a lot of call testing I have come to the following conclusions: 1. Skype calls (callout or callback) on n900 have appalling voice quality.
2. Incoming Skype calls are _ALWAYS_ dropped. I have been able to receive an inbound Skype call so rarely that I have begun considering the possibility of taking off my Skype In number from GV.
3. Gizmo callbacks have astonishingly good voice quality. If I don't want to use my tmo minutes for dialing out, I always go for a Gizmo callback.
4. SIP call-out (the "sofia" option) has not worked for me. The GV female announcer tells me that this call could not be completed at this time. This may be Gizmo specific - I don't have another SIP account to test with.