50-person crew, all men: can urinals replace some toilets for OSHA?

Yes, urinals can stand in for some seats, but with 50 guys you won't actually save a seat.

The footnote under Table D-1 in OSHA 1926.51 says urinals may be provided instead of toilet seats only where women won't use the facilities, and the seat count can never drop below two thirds of the minimum. Your minimum is two seats and two urinals. Two thirds of two is more than one, so you still keep both seats. The swap only starts freeing up a seat once the crew is big enough that the minimum is three or more.

I'd also be careful betting on "all men" for six months. One female inspector, sub, or new hire and the urinal substitution is off the table. I've watched a super scramble for an extra unit mid job because of exactly that.

On the freestanding question, the table counts urinals. It doesn't spell out stall versus freestanding, so I wouldn't read anything extra into it.

For accessibility, the ADA cluster rule is about portable toilet units clustered in one spot: at least 5 percent, and never fewer than one, have to be accessible. A urinal can't fill that spot, so plan on one of your seats being the accessible unit if the rule applies to your site.