RE: ELN60-27 - http://www.powergatellc.com/mean-wel...er-supply.html
click on the model you are interested in, their prices are quite good.
With the ELN series, if you leave power to the driver but unplug the 10v supply, the main LEDs will likely be of sufficient dimness for your nighttime illumination needs. YMMV
I would definitely use between 2 and 4 XPGs on a 6gal. You will not be driving them remotely hard.
Also I would suggest spending a bit more on a driver and getting an "LPF" series - they can be dimmed without an extra power supply, just a potentiometer. The only drawback to these is that there is no seperate control for maximum current. ( http://www.powergatellc.com/mean-wel...er-supply.html