Division commanders and in command aren't the same thing. On WB's ship WB was commander, Marco being 1st Division commander means he's the 2nd in command, the next in line to take command should the commander not be able to. Aizen wasn't an Espada though, Dragon is a rev it's not a relatable comparison. Aizen created the Espada's life Dragon didn't create Kuma, Sabo Ivan etc. You're making the terms 2nd in command and 2nd division commander equivalent and they're not.Oda has been trying hard to portray Sabo as a post timeskip Ace. Ace was Second Division Commander. You would think Whitebeard would be number one but Marco had that role. Instead Whitebeard commanded the whole crew. Dragon is the same imo, there is a number 1 in the Revolutionary Army after Sabo but then you also have Dragon who commands the entire revolutionary army. Think of it as the Espada from Bleach. Aizen was stronger than all of them but he was neither a nr.1 or nr.0, he simply commanded the whole group. Luffy had a small crew untill recently so you can't really compare the two. Also it'd be lame if there was no one between Sabo and Dragon in ranks.