You may very well have sufficient filtration, but your 120 is borderline size wise, and the rest are just to small. Your clowns will host with time, very few jump into any nem immediately.
I'm not telling you not to do it, I just think it's a very bad idea, and will be a very costly lesson in the end. What it boils down to is, your 120g is borderline to small. They rest are too small period the end. Carpets are tough to keep to start with, they are very poor shippers which translates in very tough to get a healthy specimen. A LOT die shortly after arriving at your lfs. Hadonni's are the easiest, they only get tougher with other species.
As a test, take a piece of plexi, cut a 2ft circle, and try to place that in your tank. You'll be giving up half your tank just to that nem. Does your rockscape even support putting a nem in? Is there room for it to safely set it's foot in the sand? What about how your rock and how it's stacked. Is it stacked well enough to take a nem burying it's foot under them without collapse? Mine moved entite stacks of rock when they decided to move. Then comes the issue of a cramped tank, and your nem now eating anything it touches. That includes clean up crew, a fish that rub up against it, anything really, I found emerald crabs to be very bad....they wouldn't stay away and always wound up dead withing a week. I literally had a graveyard of shells and critters that fell prey to it. They are stickier than duct tape, and their sting is not very friendly....unlike a bta which most humans don't' even feel. A carpet will wrap itself around you, and then you have to make the decision of possibly killing the carpet trying to peel it off, and wait until it decides your not food and lets go....all the while suffering the effects of it's sting. You will get hit by it eventually, we all do no matter how hard we try to stay away.
After having mine for around 2 years I think it was, and what I learned, what your thinking of trying is doable, but your certainly not giving yourself the best chance to keep them alive long term. I personally feel that you'll hate it within months.
These are my experiences and recommendations......take them or leave them, but please keep us posted on your project. I love clows and wish I had room for more tanks just so I could keep more than the 2 pairs I have.
Some pics to prove it......tank pre nem......notice how high the rock is stacked. I took a lot of time making sure everything locked together.
Tank with both nems....notice how much the tank changed due to the nems knocking stuff down. Here they are 18" or so big. I don't have any pics of when they were 24"
Individual pics of the nems.
and here's what your tank looks like when they decide to go for a walk. Notice in all 3 pics the purple one is in a different spot.