Congratulations on the first egg! I do want to make sure you have removed the extra hen, for her safety and for the safety of the breeding pair, eggs and chicks that may hatch. Your pair needs privacy so that means moving the extra hen away from them, and keeping yourself away as much as possible. You need to try to only be around them for a quick check and when you need to feed them. Do not clean the cage or the nest box during this time. Anything can disturb a breeding pair, and new parents are especially sensitive. Too much checking on on the eggs by you can result in them abandoning or eating the eggs.
I hope you are feeding them a nutritious diet as I described in the previous response. A small egg can be a result of poor nutrition. If the rest of the eggs are undersized, they may not be any good. And it may mean the hen needs a lot more protein and calcium. A soft egg is a bad sign and means the hen is severely deficient in calcium. Hopefully the rest of the eggs will be normal. And if you have never seen a cockatiel egg before, it could be the egg is normal and just smaller than you thought it would be.
The hen will lay an egg every other day. It sounds like she has started incubating the eggs already, but some pairs will wait until they have a few eggs before beginning incubation. This keeps there from being so much size difference between chicks. In your case, if she lays the normal 5-6 eggs, and if they happen to all hatch, the first to hatch will be a week older than the last to hatch and this can be a problem. Sometimes it is necessary to pull the older chicks to hand feed so they do not crush the younger chicks. Each egg takes at least 48 hours after the hen starts to sit on them to begin to develop. I do not recommend that you check the eggs for fertility or disturb the parents by checking on the eggs. This is their first time and in most cases, the first clutch will fail either by not being fertile, not being incubated correctly, getting damaged or eaten by the parents or if they hatch, the parents fail to feed the chicks. The risk for failure goes up the more that you disturb the parents. Even if you catch them both out of the box, checking it can make them rush back in and be defensive and often eggs get broken when this takes place. I know this is an exciting time for you, but as I always say, it takes a lot of patience to breed birds. Limit nest box checks to once per day, to confirm if she has laid another egg. Once all eggs are laid, stay away until the eggs start to hatch. Then one or two quick peeks during the day to make sure the parents are feeding the chicks. If the parents are very defensive and lunge at you or strike at you, then stop checking the box. Defensive behavior like this can result in the eggs getting destroyed or any hatched chicks being bitten by accident. If the due date goes by with no hatching, wait at least a week after the last egg laid is due before you remove the eggs and nest box, and then rest the parents for a few months before giving the box back to them.
Good luck and thanks for the update!