An Ergo is definitely not a must have! Sounds like you've got a GREAT stash already.
Moby is great for the newborn stage, especially when it's cooler outside. It's a snuggly carrier. I recommend watching you tube videos if you haven't already since I found it overwhelming once my DS was born. Just a warning, it's common for people to find that it stretches too much once the baby reaches 15-20lbs. And please don't ever attempt a back carry in it (duh, right?).
Ring slings are great for newborns and they are still my most used carrier. I can't wear it for long periods of time, but I can get through a quick shopping trip or through church service with one.
I have a Mei Tai Baby and I love it! I really love how the bottom snaps down because that was so helpful in the early days. I still use mine. I'd probably use it more if it wasn't 1,000º outside here (silly me bought a black one while living in a desert lol). That was my first carrier I used for a back carry.
About the Ergo, I do own one. I wouldn't get one for a newborn. It's okay in a front carry. I pretty much only use mine for back carries. Right now DS is around 29-30" and 20-21 lbs. Of all our carriers (I own quite a few ), it has one of the smallest bodies. DS can easily be arms out in it, which he likes but I do not (hair pulling). He is also already knee to knee in the carrier. A couple of more months he he won't be supported knee to knee (ideal positioning is support from knee to knee with butt lower than knees). It's not a must to be knee to knee, but I wonder if a year from now I'll want to ever bother with it. I know I'll still be wearing him then too! I do love having a soft sided carrier around for my husband's use though (he hates having long things to tie, though he has used my mei tais when I'm around to help). I also like having SSCs around for when I'm out of the house and I don't want anything dragging on the ground. I have an Ergo and a SV baby sized Bamberoo (which DS isn't close to growing out of yet) currently. I've tried on a few others and used to own an Angel Pack LX. I really loved the APLX, but I couldn't justify keeping it around since I own so many other carriers just for one kid. lol.
Sorry I rambled; I love to talk carriers and this board is practically dead.
While an Ergo is not a must have, it is, by far my favorite carrier right now. I absolutely loved our Moby wrap for the early days and our Rockin Baby ring sling but my 20 month old is almost 30lbs so the Ergo is the most comfortable way to carry him now. I have a mei tai and I love mei tais but Xander prefers to be on my back and I haven't mastered the ruck carry (lack of trying since I got my Ergo lol). It's not a must have but if you get one, you won't regret it Babysteals is great but if you search around for blog contests, you may be able to win one- that's exactly how I got mine.