your option:

1. use ac21 b4 gc--no problem but could have a little bit of hassle b4 gc

2. leave 6 months after gc---no problem
3. leave right after gc without a big fight--no problem for gc itself, but some problems could occur if you want to apply citizen in the future

4. leave within 6 month after gc due to laid-off, firing----no problem at all.