so many variables... first off can you take and post like 3 or 4 good pictures of your grow space, a few of both your veg area and of your flower area, it will help greatly in offering help.
but i can say this, having the heater at the same level as the plants is not a good idea, personally i dont need a heater but i have my light up almost as high as it can go, and ill use over-turned large pots or crates or even a table, anything to adjust each individual plant up or down to keep all plants canopies on an even level with each other, so you move the plants not the light, and having a heater if needed on the floor below the plants would be alright, better yet if you lets say have a grow tent and you need a heater, id get one that could heat up the whole room to the proper temps needed so when the exhaust fan is pulling air through the tent its pulling the air in the room into the tent and out lets say a window so the warm air isnt circulating back into your grow room...
personally i use a closet with the bifold doors taken off for my veg space, and a AgroMax Large tent 55"x55"x79", with as i said my lights (600w hps and 2 135 watt 7band LED ufos in opposite corners from eachother) at the top of my tent so i raise or lower my plants to keep the canopies about 20-15 inches from the bottom of the light hood... i can scooch them about 10-12 inches from the hood if i really wanted to since i have a good exhaust fan so it keeps the lights temps down really well... but i prefer the 15-20 inch spacing so the LEDs done bleach the tops from being too close...
again it would really help if we could actually see your grow space so we can better offer help more tailored to your grow spaces as opposed to general rules of thumb...
hope that helped a bit, also when to flower is dependent on personal choice, grow area restrictions, and strains, if the plants are far more sativa dominate then youll need lots of room for stretch and added flower growth so you may want to top and LST the plants, but if they are far more indica dominate then you might not need to top and just let them get about 1-1.5foot+ tall and lollypop the lower garbage and just let it go all natural so to speak... again there are many variables with the strains, you can have an indica dominate strain that doesnt really grow with a typical indica short and compact structure because it might be a hybrid and is mostly indica in general but its growth structure resembles its sativa parent, so its taller and stretches more... kind of a winging it situation until you get the hang of things with that strain...