1. Does it only apply to uhh.. human-to-human relationships.. could it apply to human-to-animal/plant relationships, even to human-to-item ? Or perhaps some offspin or what not of it?
It only applies to human people, as I understand it. Robin Dunbar examined why certain high-functioning human groups peaked and split around a certain mean number.
2. Does the number say 150 mean 150 strong relationships (but takes a lot of time to maintain), but lets say different groups of 20 strong, 30 semi-strong, 100 average (based on the amount of time put in with different relationships/people).
I assume from this question I did a poor job of clarifying this in the article itself. 150 refers to how many intense, intimate, high-functioning relationships you can maintain in your network, an activity that requires high-amounts of social grooming to pull off.
Everything else follows from that...with the mean limit of 150, you can have any number of any level of intimacy, as long as you put in the work to pull it off. How does it break down, once you start investing less than required for a high-functioning and intimate relationship? I don't know that it even works linearly. I see it as entirely possible that you'd have to work a certain amount before you get into someone's in-group, and until you get there, they never really trust you. Once you get there, you've made it.
So the theory itself doesn't talk so much about this part of it. I do know other folks have looked into this kind of thing, the different levels/investment in intimacy over an array of relationships. You might consider posting it here if the mood strikes you to google it.
I suppose that angle on the Dunbar's Number issue doesn't interest me as much the basic insight of fundamentally changing the orientation towards what we want from our relationships and family. But I don't know, I have more to learn about all this too!