USB CDROM!
I must have missed that on the first page. Ya, older servers won't like that. And you'll probably find the same with the pen drive too.
I'd go to the closest PC recycling place, buy a $5.00 IDE CDROM, pop open the case, plug it in, and install from that. Even cheaper if you can scavenge one from another computer for a short time.
More than likely these servers were originally imaged or PXE installed, which is why no CDROM drive. Didn't need one.