definition of short stacked on the bubble

Hey guys, I've started playing a lot of 10+1 SNG's using Aleo's guide and need advice on my bubble play. Aleo's guide recommends pushing if you become short stacked provided you are the first in the pot.

I'm curious as to what exactly a short stack is. I usually push if I have 8-10 BB's, but I feel i might be being too aggressive in instances where one player has a dominant stack and their are two other very small stacks. Say one person has 4500, I have 1400, the third has 1200, and the fourth has 900. If blinds are 150-300, am I right to push with a pushing hand or should I play more cautiously?

thanks