Parallel charging LiPo packs has become very common in the RC hobby. Granted, there is not a lot of empirical evidence about how good or bad this is. Only the fact is that a lot of people do this on a daily basis.
Personally I have been parallel charging 6s LiPo packs for 2 years now with good results. I have some budget packs that are over 100 cycles, so in that regard, I am happy with the life I got out of the pack.
My parallel charging routine was not very stringent. I likely never charged packs with over a 0.25V/cell difference during this time. I think in the future I will be more careful about the voltage of the packs. My recommendation would be to stay under 0.1V/cell difference.
The problem with parallel charging is that it is quiet easy to make a mistake, and connect packs of dissimilar voltages. So if you are going to do this, I would always double check your pack voltages before connecting.
A very good resource on parallel charging can be found on the Tjin Tech site. This is a very thorough examination IMO. If you scroll to the bottom it also addresses the potential surge currents when connecting the packs, and also includes experimental measurement of current that is released at the initial connection.
Having used the Sparkfun battery charger you listed (and in general designing with the Microchip MCP73831 battery charger), I have found that while it is supposed to be be safe, the Sparkfun device will get alarmingly hot when charging a large, almost empty battery from 5V USB power. I suspect this is simply due to the linear nature of the battery charger and the small un-heatsinked package., but I would be a bit weary of leaving it plugged in unattended because of this.
Best Answer
500mA is not enough to charge that battery in a timely manner (24.8Wh/(0.5A*4V)=12.4h), and that's if you have it connected to a device that it can negotiate with. Consider putting together a current-limited switching regulator with a power MOSFET that can push at least a couple of amperes to the battery (or, you know, just buying one).
But don't put multiple chargers in parallel since even small differences in output voltage could cause damage to either or both chargers.