If you would:
http://www.cdrinfo.com/articles/svcd/
is _the_ link describing the official specs...
In brief: audio: between 64 and 384 kbit/sec, video: up till 2600 kbit/sec, however: total bitrate should not exceed 2.8 Mbit/sec. It'll also explain the whole thing with the svcd scan offsets, and those infamous underflow errors you can get with bbmpeg muxing.
Something I wonder: this document states: audio bitrate: may vary from frame to frame. Does this mean you can create variable bitrate audio for svcd too?