Hey there topcat!

I've been growing in a tent for quite some time now and I can relate to your issues. From what I can tell - there's a few suggestions I can give:

1. When your plants are as small/young as they are, you might want to consider starting with a few floro lights close to your seedling instead of the big 600 just yet. I use power-bars with those plug-in light sockets and the warm/cool bulbs to start my plants off. The 600w light is drying out your peat-cups way to fast from the heat. Turn off the 600 for now and go in close with some small CFL lights to start.

2. Since you're in a tent, you want to get some 'negative' air-pressure going so that your tent walls actually get 'sucked in' from your fan exhausting the tent - then be sure to have passive intake holes at the bottom of the tent to allow fresh air to get sucking automatically... that will bring your temps down quite a lot.

Question - how big is your tent?

I hope that helps - please let me know if you need more help! =)