I'm going to weigh in on the red blood, thicker skin side. I agree with what was said about heme based blood being necessary for successful crossing with other races of heme based blood (setting aside virtually every other
scientific detail about hybrids, but we are trying to accept things as established in Trek). Cardassian tongues are also pink, and I seem to recall the times Garak got beaten up, he had red blood. It looked dark because of the lighting, but it still looked red.