I can't speak to the C02 part of it because I don't use it so I don't know if it's necessary to slow it down.
But the only other 3 things that come to mind are
1) based on how much water fish are usually packed in, 10ml really isn't a large percentage of that, so you might want to up the number of water additions you use. The basic idea of the water adding process being that you want to get the fish into as close to 100% your tank water as you can, but slowly. Many people will add small amounts several times over an hour or so, remove half of what the fish are now swimming in, and repeat the process, so that by the time they are netted out, they are swimming in a much higher % tank water than store water.
Personally, I am a fan of the drip acclimation method (nothing fancy, just tie a knot in some airline tubing) i've drip acclimated sensitive fish for 4+ hours and I've lost very few since starting to acclimate this way. I added 7 glolights to my 20 a while back and I ended up losing 2 which I was confused about so I brought them back to the store with a water sample, it turned out my pH had nose-dived and was at 5! So the poor fish went from a pH of 7.5 or so to 5, I'm pretty sure the only reason I didn't lose all of them was the acclimation.
So that's one possiblity
2) is simply that it's not good stock. I've heard harlequins are really hardy, I've also heard of people who have had trouble with them, this probably likely due to stock variation
Which brings us to the last, least likely, though also least fixable issue that I've encountered...
2) Unfortunately, this one is less fixable and let's hope it's not this reason, but a couple years ago i had a tank that i simply could not add new fish to, they always died, except the 2 little platies that had grown up in it. Best I could ever figure was some disease the platies had learned to live with. They were always stunted though and died young. I ended up having to tear the whole thing down, bleach it, scrub it and start fresh. It sounds like you have some survivors and the other fish are doing well though, so I'm guessing this is not the problem.
That's all I've got. If you want to try and get your harlequins up to at least 10, I'd buy a few extras just in case, and when you acclimate them, I'd probably try adding more of your tank water to the store water. Not more at once, but a larger number of small water additions, or you might try drip-acclimating.