Actually, the answer to your question is yes. In fact, when switching from one formula to another, many doctors and children's books recommend a slow switch in which you mix the two formulas. The important thing to remember is using the right measurements for each. Not all of the scoops are the same size, so make sure you are using the correct scoop for each formula and not mixing the two formulas together into one can and then scooping it out. For instance, my 3 month old was having some problems with the regular formula so we had him on the gentle formula. We just recently switched him back to the regular formula. He drinks 6 ounces at a time. So when we switched him, we used one scoop of the regular and 2 scoops of the gentle for a week and then 2 scoops of the regular and 1 scoop of the gentle for a week and then all regular. Also, make sure you are using a full 2 ounces of water (given that she is 10 months, I'm sure you already know this but just in case anyone out there is new to this and reading this). Don't put the water in the bottle after the formula unless you are measuring the water in another bottle or measuring cup. If you put the formula in first, you may not be putting in enough water, otherwise.
Also, I wanted to mention that my kids had problems with the similac but not the enfamil. The Sam's Club and Walmart brands are made to be comparable to the enfamil. All formulas are regulated by the FDA. They all must follow a strict guideline as to what is in them. The store brands (Walmart, Target, Sam's Club) are all just as good and healthy for your child as the name brand. You might want to try the Walmart or Sam's Club brand if money is a problem. They are a lot cheaper and work just as well. My daughter and sons have thrived and are thriving on them. (My daughter is 8 now and my sons are 3 years and 3 months). Even though I knew they were governed by the FDA, I still compared the labels to make sure and I have to say, pretty much the same.
Good luck with your little one.