I'd love to hear a post from you about taking foster kids that are older than your biological kids, as it seems from your blog you have done that in the past. Did that work for your family? What are the things you did to help your bios adjust? (I've got 6 and 3 year old daughters. 6 year old is a total alpha dog type. The foster child would either need to be at least a year younger or 5 years older and preferably opposite gender. And I'm not very confident that even 5 years older would work. 3 year old is very flexible, so I don't worry much about ages versus her.)
You've gone straight to one of the big questions in fostering, namely how the placement fits your own family.
Sounds like you're more than halfway to the answers in that you have a very clear picture of your own family and their strengths. And a clear picture of what would work best for your family, which is brilliant.
I think the first thing to make sure you do is drum it into your own children that you are not taking in other children because you are in any way unhappy with them. Might sound ridiculous that they might think that, but they might, deep down somewhere. You need to tell them that you're going to go into fostering - with their support - because they are so wonderful they can help you do an amazing job. Give them a sense of ownership of the whole project, and make sure they feel the same rewards you do. Praise them endlessly for their kindness and tolerance.
On to your first specific question about having foster children who are older than your own biological children. In my experience it's all down to the individual children involved, and that's something you try to hammer out with social workers before any foster child arrives, and believe me they take it very seriously. If an older foster child fits they'll bring dividends to your children as well as themselves. If an older foster child doesn't fit and the placement upsets the carer's own children, it's not going to work and everyone wants to avoid that.
You say your 6yr old is "alpha dog type", which is very colourful but not quite enough to be specific. So; as you appear to have confidence in your 3 yr old, let's focus on your older child.
I wonder if you have a moment to elaborate on your eldest daughter? (Being careful to respect her anonymity of course).