Always best to put rubber or ideally machine felt between item being moved and skates. I’ve moved loads of things over the years even on rough concrete outside and had no problems .Meta on metal is where most go wrong
Concrete has about 65 percent of its strength at 7 days and i think about 90 percent at 14 days. If you assume 75 percent at 10 days with 3000 psi concrete you would need a minimum of about 6.5 square inches of roller area. Seems like four skates would exceed that.
However, if the contractor adds water to make the concrete flow better the strength can be significantly lower.
For my Hilmans I purchased some really thick closed cell rubber that is used for cushioning, like anti fatigue matting, I think it's probably 3/4" thick? Anyway, place one square on each roller and when one gets a little unloaded it will still tow along nicely., instead of just staying behind. They really transform the experience if you have something heavy enough to need 4. If you can get away with 3 it makes it really easy to steer the equipment as it's being put in place.
I know this isn't possible with everyone, but in my shop where it's tight in places I put an heavy anchor bolt in each corner of the shop floor. This way I can use lever chain hoists with long slings to grab onto whatever I'm moving and move it very slowly but steady with minimal effort. When it's 40,000 it's nice to just be able to move it exactly an inch when that is all you want, in exactly the direction you want. I love the steel roller Hilmans, they take the least effort when things get pretty heavy and you can actually roll them over things like expansion joints. A buddy of mine tried using the nylon version when he moved his Mori NT and they squashed with flats instantly.
I need a better "toe jack" however, but that is a different conversation...