I borrowed a 55t big ring from a teammate, but do not have a 42 inner ring to go with it...only a 39. Are these compatible?

it should be but you might have to make a small adjustment with the fr. derr.
If it is Shimano they recommend no more than a 14 tooth difference between the two rings but I do not believe you will have a major issue with your shifting

i think its too big of a jump from big to small (16t)

shimano likes 13 tooth differences

except that on their FC700 compact crank, where it is a 16 tooth difference. Works perfect with a shimano FD - they say so and many who have it agree.

From what I understand, a 16t jump is the maximum rise engineered into both 110 and 130 BCD cranks and applicable derailleurs, so it should work. As HAMMER MAN said, you will most likely need to raise the front der a couple mm's because that 55t is a pretty big ring.

Do you have massive guads or do you just ride downhill a lot?

doing a downhill sprint or a TT with thighs that look like this?

^^^ Those look like my legs.