i'm going to do the same, i just assumed it would be best to fluctuate the bottom heat.

also how old should trich seedlings be before you take the heat off all together?

i'm pretty new to this, but i think betty has a post saying that she hardens hers of at the 3 or 4 month mark, so depending on how old they are wait util melbs warms up a bit and slowly start to harden them off.
but hey i'm new to all this too, have about 500 seeds down but its my first go, lucky theres so much good advise from the experienced guys on this forum.
good luck

Got that right SP I gradually open the bags and as soon as its warm enough I turn off the mat and get them outside in the hothouse of somewhere where they will get morning sun. Gradually of course with the sun

Good luck with your seedlings

In regards to the globes I think it would be better to use a combo of cool and warm globes, ie. high and low kalven. This scale in regards to globes refers to different waves lengths of light I think. Cool light is usually 2000K, a warmer globe might be closer to 5000K (light blue like a butcher light).
What are your guys opinion on that?

In regards to the heat mats I would have thought to turn them off in the dark hours as I thought that cacti do their growing when its dark and colder, to simulate an environment like a desert with no vegetation.
I havent got into heat mats yet so I might be wrong, would you agree with that though?

If you turn off the heat mat in the dark hours many of my plants would probably freeze. With frosts the shed gets verrrrry cold in the night. I have had plants that have had their tips burnt when I did not have the lights set up correctly as it must have got below freezing. How I hate frosts, we have had 5 in the last 6 days.
The lights are on a timer giving them darkness they need for growth. Set to come on at 4.30am to help with warmth on frosty mornings.