While I was at bunnings I saw they had silaflex for installing windows. I've got new rubbers aswell. Is it easy enough to just run a bead around the window frame and press the window in or should I just pay the extra 50-100 bucks to get a pro to do it?

i only use windscreen mastic or glazing urethane.

That Sikaflex stuff might be ok if its specifically designed for car windows. Sikaflex make some good sealants.
The stuff i use on old rubbers/seals is called butyl mastic 'black'. Its cheap and remains soft so its easy to remove the rubbers later.
In saying that if you have a new window seal it shouldnt need any sealant...

Won't the windows just full out if I just push the windows in?

As Nick said with new rubbers you won't really need sealant. The rubber has a seal lip and a lip that goes over the metal window opening on the car body. However with these types of rubbers new or used I normally put a bit of sealant in each corner just incase the seal does not make full contact with the body.

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1928 Model A CC Pickup

dont use the sikaflex on a window with a rubber mate it sets hard you should only use mastic on screens with rubbers

They have 2 different types or window sealant 5614 and 5615 which one to use?