Should I charge an in-series 6-pack of Li-ion batteries at 4.2v 1A or 25v 1A?
Since you have 6 Li-Ion batteries in series, you could charge them at 25.2V (Since the max charge for a Li-Ion is 4.2V * 6 = 25.2V). The current used the charge the cells depends on the battery itself. Batteries usually have a part number printed on them. So if you find the datasheet, then you can determine the maximum current you can charge the batteries at. It will usually say something like "max charging current is 0.5C" with "C" = the mAh rating of the battery.

However, when charging batteries in series, it is always recommended to use a balance charger. This is to ensure that all of the batteries are at the same voltage potential and no single cell becomes over charged.