You'll want to use low rates (smaller surface movements) when you want your plane to be less agile/sensitive, such as on take off and landing. High rates make your plane more sensitive to stick input, more agile.

Cycle your batteries with a "smart" charger a couple of times (if you have one), and see how much charge they will accept. If their health has deteriorated considerably, get rid of them. If you have no way of determining their health, spend a few bucks on new batteries --- no sense in risking your planes just to "save" a little money.

Yes, you can use the same fuel for all of them.

As for rudder/aileron mixing, I personally see no reason for it, but someone else might have an opinion on that.