caliper question

I have an '02 ALero ecotech 2.2.

It never occured to me to change my brakes by using bleeder screws instead of a C-clamp. I tried it with tragic results. I broke 2 bleeder screws... no fluids came out so I scratched the new tecnique and went back to the C-clamp. I have one broken bleeder on the back and one on the front. When i do my next brake job, I am considering replacing the calliper inthe front and then when i do the rear, replacing the rear. I am too gunshy to open up my bleeder screws after my disaster. So here's my question.

Can i bleed just one calliper or do all 4 have to be bled or do i bleed per axle?

Since you plan on replacing a front and rear caliper bleed all four per a proper brake bleeding procedure.

Also, next time consider using some PBlaster penetrating lubricant first before opening up the old bleeder screws if you live in an area subject to high rust and/or corrosion.

these bleeder screws are like plastic or something... i used pb blaster and they still broke. So I'll do more harm than good if I don't bleed all 4 at once?