A. In general, different brands of the same type of formula have the same basic ingredients.
For a milk based formula, this includes that lactose is the carbohydrate and they have cow's milk proteins. There are small differences though, including different combinations of whey and casein proteins. And the fat source in the various milk based baby formulas are provided by different types of oil.
These small differences likely won't be noticed by most infants, so the big issue when using different formulas, like she is doing, is usually taste. Some children are quite particular about what they eat and may notice the taste differences of the different formulas and they may not like it.
However, if you consider that the taste of breastmilk likely changes depending on what a breastfeeding mother eats and these children usually do just fine, taste likely won't be a big factor for a baby drinking different formulas.
Of course, if she were particularly more gassy after using one of these baby formulas, then it should be avoided. But if she really is 'doing great,' then this is probably okay to do.
Still, there is no big benefit to using multiple brands of formula and with a little planning, even if cost and convenience is how she is choosing which formula to give her baby, she should be able to stick with one brand.