For ease of use in a practice situation you probably only want one unit as a limit. <PERSON> wrote a nice answer based on force, I will try to do the same from an energy perspective.
So I imagine the safety involves not being crushed by big crates. For fingers and such complete other rules apply, as well as for sharp or tiny things (You can't catch a bullet).
So if you are are in danger of being crushed by a crate hurling towards you in zero g you have your arm's length (about 1m) to catch the crate and slow it down to a complete stop. If you can benchpress 80 kg on earth you exert a force of 800 N if you use a gravitational acceleration $a$ of 10, according to,
$F = m a$.
The work needed to do this can be calculated with,
$W = F s$,
where $s$ is the distance along which the force is exerted and $W$ is the work done in Joule. So bench-pressing 80 kg in earth's gravity costs 800 J.
So 800 J would be the energy limit for objects
So for moving crates in zero g only the mass and velocity matter since they don't experience force until you push against it. 621
So the energy (here we will assume work is equal to energy, basically ignoring the directional component of work) of a crate can be calculated with,
$E = \frac{1}{2} mv^2$.
So you can calculate what the maximum velocity of a crate is allowed to be depending on the mass. In practice a table with a certain safety factor involved is probably better. So some safety limit speeds of different masses can be:
Mass (kg) Max Velocity (m/s) Max avg. Acceleration (m/s^2)
8 7.1 100
80 2.2 10
800 0.7 1
As suggested by <PERSON> I have added the maximum average acceleration a worker should apply over 1 m (arms length) to the crate. This is all assuming the worker has some way of counter balancing the force.
So for working safely you would need to remember some numbers and be able to judge the velocity of an object. It works for other gravities as well as long as the force direction of gravity is perpendicular to the plane the crates are being moved in. 771