I have 4 bulb T5 fixture over a 20g nano and have never had any heat issues. I do run an open top and the fixture has fans to take the heat away. I think you may have a heater problem if you are have temp troubles using an LED fixture, or maybe you have a closed top?
I wouldn't be concerned about the watt output of a 6 or 8 bulb T5 fixture, if it is initially to bright for your corals you can elevate it and gradually lower it over time or put layers of plastic screen over the tank and remove them one by one as time goes on. I suggested this many bulbs as it will give complete coverage over a 24" wide tank, you can keep any type of coral you want and it gives more choice on bulb colors. There are many posts regarding T5 bulb selection and lots have pictures comparing the different set ups. A lot of them use various white and blue combos and then add a Fiji Pink bulb to bring some of the reds out better.
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