I did a 1 Gallon Pico with a Coralife 50/50 10Watt bulb, and an AQ Mini HOB. No heater, the HOB and light is enough to keep the temp between 79 and 81 (day/night). It's been running for a couple months, no huge growth in the Corals, but nothing got dead.

http://img.photobucket.com/albums/v2...n/48fcbc68.jpg
http://img.photobucket.com/albums/v2...n/7348a31c.jpg
The HOB is full of LR rubble, top up done every day or two, 20% WC at least once a week (with a turkey baster, takes like 2 minutes). If you go bigger, use a bigger HOB, and maybe a small powerhead. If you get the right sized equipment, you won't need a heater, the pump motors take care of it.