The light color and plumpness are common for babies. I would expect a darker color to develope as they mature a bit, maybe after at least 7-8 months. Over all they look very nice indeed. Generally they seem to resemble grown plants after the first half year, and I consider mine out of baby phase by one year.
As for the heat mat, I primarily use that for germiination or in winter when the room gets below 50f at night. The heat tends to accelerate the drying of the soil, so keep an eye on moisture as you leave the dome off more frequently. My only problems arose when I left them for a week with the heat mat on, and of course Bridgesii come from a wetter mountain place than the desert cacti.
Please tell me about reptile calcium sand. Never heard of it or it being a problem, but would like to know just incase. Good job my friend, nice seedlings.