It's actually best to filter a tank at twice its capacity, so long as your fish aren't being blown around. Thing with any filter that has a varied size range like that is it's probably only decent for a 20ish gallon tank, plus they often test those filters without filter media. In short, get another of those filters, at least, and that should work.
For the 10g, either use two of the 10g filters or use the one you have on the 30. Don't lower the setting unless fish are getting pushed around a lot. If you have one high-current spot, that's alright. It's very hard to over-filter a tank.
Test your water now and then... Get onto a water change schedule where nitrates stay at an acceptable level, and you're good so long as nothing freaks.
Acceptable levels for nitrates vary wildly on who you ask, but under 20 seems to be a good number.