I am not quite sure that I perfectly mentally grasp the concept of cross-rhythms...A few people at my school have tried explaining it to me...I am highly suspicious that some of these things are involved in certain pieces of guitar related music that I have been trying to transcribe for a little under a year. My efforts have been yielding for the most part terrible/in-concise results )-:

No, Poly-rhythms are a different animal entirely...4/4 against 3/4 is a poly-rhythm. I asked my piano teacher about it earlier today and he sorta helped me sort it out the nature of cross rhythms. In a nutshell, the general formula for cross-rhythms involves an (X/Y) relationship where X=the number of notes in a cycled phrase/ostenato and Y=# subdivisions/beat (so if its sixteenth notes, Y=4)

4/4 B minor

In the context of a straight sextuplet groove, this idea when cycled would constitute a 7/6 cross rhythm

E|--9-10-9-7-9-7-10--|
B|-------------------------|
G|----------------------o-|
D|----------------------o-|
A|-------------------------|
E|-------------------------|

We were not 100% sure of this conclusion but we are fairly confident it is on the ball.

X = # 16th notes that make up the phrase:

For instance say you were to loop this legato phrase while playing each note as a 16th note (say you were noodling over a f#* chord)

B/c you are playing each thingy here as a 16th note, Y=4...And because you are playing a total of "7" 16th notes here before you are back on the F# that you began on. hence each time you play this phrase, you are full-filling 1.75 beats per/cycle

in this instance X=4 and Y=7 hence it would mean a 4/7 cross
|--------------------------|
|--------------------------|
|------5/11-------------o|
|-4/7------------10\---o|
|-----------12/9---------|
|-------------------------|

Dangit typo* alert I fliped it around...not a 4/7 cross meant to say 7/4...sry /-:

7/4 b/c there are a total of 7 notes/cycle and you are playing 16ths which in simple meter are 4 to a beat. hehe