I agree, iron balusters should have a pattern in them. As bp pointed out, the difficulty is that you've got some stairs with 2 and some with 3, so making a regular 4-baluster diamond shape is problematic.. However, they also compounded the randomness by cutting some balusters off at the top and some at the bottom. If they cut them all to length off the bottom, then a pattern would emerge, though not a diamond. Look at your first photo above. The doubles are all the same distance from the rail to the top knuckle. So a straight line connects them all the way down, and that line is parallel to the bannister. The singles, on the other hand, are not. Look at the lowest 4 singles. They are all different distances from the bannister. Try taking some blue painters tape and putting pieces on your existing balusters to represent where the knuckles should be. Try just changing them so they're all the same distance from the knuckle to the bannister and see if you like it. If you don't, then play around with another pattern until you get one that's pleasing to your eye. I would probably avoid using the blanks and just go with a running pattern like single/double/single/double. Here's what we did. On the stair and the balcony sections even though we used similar pieces, we used them in different patterns.

A quick Google search for me turned up many sites and I have no affiliation or feelings for any of them but here is one. Here is a link that might be useful: Iron Ballusters

Wood or metal? My husband and I had that question when we were planning our stairs. I looked through Houzz and saw a couple of examples of balusters that used both, so that's what we did! It wasn't so much what was in 'style,' but more, what would make it like the rest of our home - it was very US. It was the firs time he had made a stairway, and it was made in such a way that we can take down the railing, newel posts, and balusters to stain them, and stain the stairs too. We live in a 1915 house, with Craftsman tendencies.

I’m in the same dilemma with our current house (built 1985) and have decided that I’m going to paint the balusters and the base white and leave the bannister and posts stained. I looked through tons of online photos and that configuration to me seemed to make the biggest impact. We put iron balusters in a house we built 12 years ago because it was the norm for new builds and I wonder if at some point iron will start to look dated whereas wood balusters have always been around and could argue that they’re “timeless.”