I agree with Nycindie.....I would slow down a bit and try and figure out why the need for others? As young adults....being 21 is usually a natural time to explore. To find yourself, to grow and become the person you are going to become.
Really communicate and try and understand your desires behind it all.....and be as honest with yourselves as possible. At the same time, build a very strong connect through honesty and caring for each other and supporting each other....and compromise and sacrificing for each other.
I am new to this as well....and what has helped me is what I call a "Trial and Error".....I didn't intend it to be that, but it turned into that when I set boundaries I THOUGHT I was ok with (then after the experience I realized I had to re-set things because I was hurt by his actions).....Give a little at a time, or a lot at first the change and modify until you feel ok with things.
You may still feel scared and insecure.....but boundaries for me have become a safety net....."water-wings" I am using before I am ready to take the plunge
Boundaries and this family (the people her at Polyamory.com) have been my safety net
I have also learned that there is going to be some growing pains along the way...