Can you use gloss polyurethane for first coat when finishing floors?
Yes, but you better be right on the money with your re-coat times.  

A second layer of paint bonds to the first layer one of two ways.  Either Mechanically by the paint grabbing onto the last coat, or chemically by the paint diffusing into the last coat.  

"Chemically" requires that you hit very specific "re-coat windows" that are documented on the paint label/instructions.

"Mechanical" doesn't work on gloss paint, because it has a finish like glass.   

Their use of flat paint is partly to "cover" them against missing their re-coat window. 

However gloss paint can be made re-paint-able by scuff-sanding it, i.e. sanding it with something like a Scotchbrite pad to remove the gloss.  (if you're removing layers of paint at corners and edges, you're sanding much too hard).  This roughens the surface to give it some "tooth" for the next coat to bond to.  I am the master of missing my re-coat window, and that's how I handle that.

Paint also tends to be quite soft and vulnerable during the re-coat window, so unless you're Tom Cruise, painting a floor might be a bit of a problem!