Official SF Back Story: Ryu, Ken and Retsu trained under Master Gouken, who was trained by Goutetsu. Gouki (Akuma) was also trained by Goutetsu... Goutetsu taught his two students the killer arts of Shotoken, which only Gouki used... Gouken developed his own style which "edited out" the killer stylings. He passed on this knowledge to Ryu, Ken and to a lesser extent, Retsu. Then Gouki killed Goutetsu by blind siding him, once he felt that he was powerful enough... afterwards, he and Gouken battled, which ended with Gouken being knocked into a nearby waterfall... Apparentely dead (Though Retsu is still searching for his master)
Drift King EX is correct in saying that Sheng Long means Dragon Punch, but I thought it was a chinese word.... can't remember where I heard that one <IMG SRC="smilies/wink.gif" border="0">
One other thing where people get confused about Gouki's master is in SFA2.... Gouki addresses Gen as Master Gen.... this is merely a show of respect however <IMG SRC="smilies/smile.gif" border="0">
As far as game references go.... Sagat mentions the scar on his chest (And how Ryu caused it) in his SSF2X ending.... and one of the Alpha's as well.
And Ryu and Ken's master references.....? None that spring to mind. However the Statues in Goukis SFA1 ending and SFA2 background, are thought to be of Gouken and Goutetsu.
Oh, and Gouken also trained Dan.... briefly. Gouken saw that the only reason that he wanted to learn was to kill Sagat (For killing his father) Gouken only trained him the basics for this reason. Dan than went off, and created his own mish-mash of a fighting style.
Hope that clears a few things up for ya <IMG SRC="smilies/wink.gif" border="0">