You can keep the roots from cooking by raising the pots. The warmth that comes off the pad should be enough to keep them warm.
As far as topping, it really depends on how much time you want to keep them in veg and how much room you have in your space to grow them. Topping them will slow down growth temporarily but will reward you with more and bigger lateral branching meaning you'll have more consistently sized flowers rather than the traditional Christmas tree shape where the top cola is the biggest.
What are you feeding your plants? Are you using soil nutrition like in organic grows or are you supplementing with fertilizers? I use synthetic fertilizers as it allows me to supplement the plant when it needs more of a particular nutrient at a particular growth phase. I use this chart as a general guideline. The only thing I've found to be different is to start Cal/Mg a couple weeks sooner.
Do you use any Silicone in your feeds? I find using Mono Silicic Acid to be quite useful, especially if you are pruning for more branching. IIRC, a botanist told me that using MSA, if you are topping and FIMming, will work together with calcium when it is building branch cellulose and strengthen the cell walls allowing for larger and heavier colas without having to trellis or yo yo them from falling over.
What I like to do is to try and balance the extra time topping adds to veg by FIMming. FIMming is less stressful on your plants meaning the stunting window is much smaller. I'll usually top after I see node 6 forming and remove it leaving 5 nodes to break the Apical Dominance. I'll give it a week to recover and start to sprout new lateral branching. Once I see that the new laterals have established themselves, I'll FIM specific areas to try and get more branching and bud sites ready so that when I flip the light schedule I have strategically placed my bud sites where they can get as much light exposure as they can without any of the other leaf material blocking them along with filliing in the screen I use when I ScrOG. This keeps me from having to constantly be pruning away leaves.
What part of Oz are you from? When I was semi pro surfing I got to surf all over the Gold Coast in Queensland. I also got to surf at Bondi and Bells along with Margeret River, Manly and Byron Bay. Back in the day my Ozzie friends would tell me that in Australia you were either a pro surfer or an electrician(Sparky)!:hard-laugh::hard-laugh::hard-laugh::hard-laugh::hard-laugh: