What's the gap between the holes.. What's the length of the safety bar..

Gap between the holes is 1inch. Measured one center of one hole to the center of the next hole.

Which standard barbell length is suitable for this rack

1.8m rackable custom bar or 2.2m olympic barbell. A 2m standard size bar can be use also. For your information, the width for most rack are the same. 

Hi Can this use regular dumbbell weights? And are the hook adjustable? Thanks

Yes. You can use regular dumbbell weight plates for the lats machine. The J-hooks and the spotter height are adjustable.

what is the diameter of the plate can this machine use?

Both olympic and standard sizes plate can be used. 25mm, 28mm, 50mm. It comes with converter sleeves.