>>Would I be able to tag on her application as a derivative?<<

I am not sure that you make a good hedge against her bets! On the second thought, this might depend on her/your situation, and finally I think you meant to say a dependent? No pun intended there though.

Here are two starting points you might want to research further in a proper immigration law forum:

1) If you get married, the advantage is that you and your fiance will both get residency at the same time. The disadvantage is that you both will have to wait a bit longer - maybe about 3 more years. This is b/c the priority date changes from Cat 1 to Cat 3, which has a longer waiting period. Be sure to inform the imm office immediately if you get married.

2) If you get married after she receives her residency then you are more likely to wait about 5 yrs for your residency.

It's a tough call depending on your situation and circumstance. Good luck.

-JB