I do not have an IRN, a conure, or a mini macaw, so perhaps I should not be putting my 2 cents in at all, but I have been told a few things that might help.
First, I have been told that many australian birds are very gentle. Not all australian birds are gentle, but the ones you have are known to generally be among the gentle australian species. They probably won't be aggressive to a newcomer, but they might need protection from one.
South American birds are known to flock together in mixed flocks in the wild. This is one reason why they squack so loudly, because they have to be heard and distinguished by others of their species among many species, but it is also why many South American birds do ok in mixed species homes. That doesn't mean all birds from South America like all birds, or even all birds who originate from South America. It is just a general thing. This MAY mean that a conure or mini macaw would enjoy your birds, but any one particular conure or mini macaw might not.
Years ago I had both a dusky conure and a cockatiel. They got along well. The dusky was a sweet bird who never once tried to bite me or anyone else. Dusky conures used to be common in my area, but now they are very hard to locate. I do not know if they are available at all in Iceland, and not all will have the personality mine had, but one could never ask for a better bird. I believe she could have killed the cockatiel if she had desired, though. I think her beak was probably strong enough. She just never tried to be aggressive. She also made vocalizations that were not unpleasant. She never attempted to talk.
I think many pyhurrah conures (Green cheek, maroon belly, black cap, crimson belly, painted, rosifron, etc.) will also get along with your cockatiel, but everything depends on the individuals.
All of the mini macaws other than the Hahns and the Noble are significantly larger than a cockatiel. I have a friend who has a cockatiel, a budgie, and an Illigers macaw. You just have to be very careful when they are out of their cages at the same time, but that could very well be the case even with two birds of the same size. She tells me that her Illigers is very gentle with any smaller birds, and trustworthy with both the cockatiel and the budgie. Everyone I know who has macaws tells me they are gentle. I do not know. I am intimidated by them, myself.
I have a senegal and three small birds, a bourkes parakeet, a splendid parakeet, and a lineolated parakeet. I never allow the senegal to actually make physical contact with the three small birds. It is not difficult to manage this, but you have to be diligent, and you might not be able to just allow all of your current birds free range in your bird room most of the time as it seems you do now. That doesn't mean it isn't doable, but it might change their lives some if they have to be confined part of the day while you have your newer bird out. I actually would NOT recommend any of the African birds to be out with your birds, but you have not mentioned them, so perhaps you already know this.
Are pionus available in Iceland? I only ask because the bird store I frequent has the most gentle larger bird I think I've ever seen, and it is a Maximillian's pionus. I understand not all pionus are gentle around other birds, and I believe we have a member on this very site who had a white capped that bullied her other birds so badly that they could not be trusted together.
I don't know anything about IRNs, but many people on this site do.
I think that you will not know for certain if you will be able to trust any larger birds with your current birds until you try it, and you could even find that what is safe at the beginning becomes unsafe when the birds mature.
Nevertheless, even if you find that a new bird can not be trusted to be free with these birds, you can still provide a great environment for all of your birds. It might, however, mean confining the birds more than you do now.
I don't know if they are available in your area or not, but would it be possible to find another larger but gentle Australian bird? They might not be at all what you are looking for, but we had some threads going in here a while back about Princess of Wales and Regent parakeets. Again, I don't have one, have never even seen one in person, but the folks who have experience with them seemed to really understand them and were positive about them.
Oh, and one other suggestion, which might sound odd, but I recently added a slightly older bird to my flock, a linneolated parakeet. At first I was afraid to add a bird that was past the baby stage, but I found her to be the easiest bird to introduce to my flock of all of them. She was old enough when I got her that she had been through her rough developmental stages and I could tell what I was getting. Maybe if you could find a conure, IRN, mini macaw, or even a different older bird that has proven itself to be gentle around small birds that would give an even better chance that it would remain gentle. Just a thought.
