It's only 1.1 oz, so it should not affect slide function on your firearm.
The Vortex Venom 1x26.5mm Red Dot Sight can be mounted to the FNX45 Tactical. The Vortex Venom 1x26.5mm Red Dot Sight uses the Docter/Noblex footprint so you will need to use the corresponding mounting plate from FN to mount the Vortex Venom.
If your Girsan 1911 MC is optics ready, then the Vortex Venom 1x26.5mm Red Dot Sight will be compatible with the adapter plate included with your Girsan.
Yes you could mount the Vortex Venom 1x26.5mm Red Dot Sight to an FK BRNO but you would need a compatible adapter plate.
There could be a plethora of reasons as to why you are experiencing what you are experiencing. There could have been a tiny installation error, the mount could be slightly wrong or uneven, the red dot could be off or a slew of other reasons. I would recommend reaching out to a gunsmith to look into the issue as we have no service center where we can appropriately assist.
That would depend on the intended application. The 6 MOA dot is larger and would work better for self defense, while the 3 MOA would work better for target shooting.
Yes, it will.
Yes, it will if it is an MOS model. If it is not you will need a mount like the one linked below:
https://www.opticsplanet.com/burris-mount-fastfire.html?_iv_gun-model=all-glocks
It depends on the intended application. The 6 MOA dot is larger and would work better for self defense, while the 3 MOA would work better for target shooting. You would need to find a qualified gunsmith or machinist that can mill your slide.
With the correct adapter, yes!