Well the Solaris one is limited to:
as a background.
Galaxian must be the same size as it is now (or less height, width can't change), as it all pieces together to form the theme.
As for subsilver and/or any other logos, there are no restrictions what so ever.
They can be as elaborate as you like. I think for sig banners 120px high max, and 148px wide max is a suitable size.
As for content
on them, anything.
I had noticed your sig, looks very nice actually