One of the defining characteristics of Foxtrot is its improvisational nature, especially rhythmically. So whether you think of the basic rhythm as SQQ, SSQQ, or otherwise, it's actually somewhat out of character with the true nature of Foxtrot to keep a repetetive rhythm going throughout.
Some people think of the basic Bronze Foxtrot rhythm as SSQQ, because the first pattern one usually learns is the SSQQ basic. However, I tend to think of the basic rhythmic unit, even in Bronze American style, as SQQ. You can add an extra "slow" or an extra "quick-quick" at the beginning or end of the group, resulting in such rhythms as SSQQ, SQQS, SQQQQ, and so forth. So the best way to think of the SSQQ rhythm is a SQQ with an extra slow (It's also a better way to interpret the movement, in my opinion).
Certain people may argue that the SSQQ rhythm is the basic rhythm because it's prevalent in more patterns, particularly at the beginning bronze level. But if you look at all of bronze, there's probably equal amounts of SQQ and SSQQ. More to the point, there really shouldn't be a "basic rhythm" at all, because it's contrary to the nature of the style. That's why I think of the "fundamental rhythmic unit" as SQQ, onto which you can build other rhythms such as SQQ, SQQS, and so forth. This approach lends itself to more rhythmical variation, which is key to Foxtrot.
Now sometime in the history of the dance, the English evolved their own style and called it "Slow Foxtrot". Eventually as the style of movement continued to evolve, so did the nature of the dance. As a result, we don't think of rhythmical improvisation as being a cornerstone of this style of Foxtrot, even though you can certainly still break away from the basic SQQ pattern as desired.
The English-bred style (now, of course, known as International style) is the inspiration for more advanced American style, which has been dubbed "continuity style", as it involves the continuous passing of the feet. Most people (with the exception of Arthur Murray-trained students & teachers) consider this style of movement to be appropriate only for Silver, Gold & above levels. Because the movement is based more on Int'l style, it becomes less common in continuity Foxtrot to mix the rhythms up. And that's why Silver & above Foxtrot tends to drop the extra "slow" count, using mostly a repetitive SQQ rhythm.
Regards,
Jonathan Atkinson
www.ballroomdancers.com