Keeping the roots warm IS the most important thing when growing citrus indoors. Without the proper temps, roots will rot because at lower temps citrus trees cannot uptake water well. They will always have wet feet.
At a MINIMUM the root zone should be at 70 degrees, but 75-80 is better. I recommend using a heat mat underneath. I've got two Satsuma Mandarins under grow lights and on top of heat mats and they thrive. Humidity is not an issue, but at lower humidity spider mites may become an issue.
All citrus can be grown indoors, but there are some varieties that need less heat to ripen than others, which is more suitable for indoor growing. That link you posted, Dom has all the info regarding heat requirements & dwarfing characteristics. I would however stay away from a Meyer Lemon tree. They are very finicky if things are not ideal, and will drop leaves at a moments notice.
-Ross
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