I've used all these so far but banana. I've just started combinations too. I've started DS of very slow because I started him up on solids at 4 months. He turns 6 months this wednesday. We also battle constipation so I've been trying different things to help him with that too.
Yes Banana can cause constipation, I use that if they have loose stools. Pear has high fiber and I make sure I keep the skins on all of these and puree them that way so they get the fiber. My prenatal vitamins with high iron caused some terrible constipation in our sons when I was breastfeeding :( It took a while to figure it out but then once I did a week trial he went back to normal after I stopped my prenatal. Think it was the iron in them.
I initially used Pureed carrot as I was told not to start on fruits but carrot was a sweeter vegie. My DD loved it! Then I used Pumpkin and sweet potato. Shortly afterwards I used pear and apple but not too often, sometimes mixed them with other vegies. I didn't used bananas for the first few months due to constipation.
I will mention that with breastfeeding the stools are looser because it is digested rather quickly. So depending on whether you breastfeed or use formula will depend on the number of bowel movements your baby will likely have daily.
What did you use to mix your foods up with? I have been looking into the Baby Bullet instead of using our old little processor.