I think you need to be rather more specific with your question, because basically what you're saying at the moment is little more that "I've got a Mac mini, what should I do with it". The answer to that is entirely up to you, since it's your computer - and since you've got other Macs, you're clearly not unaware of what a Mac mini can do.
Thus: No, your mini can't really be used with either of the other two Macs you mention. It CAN be connected to either or both in terms of using it as little more than an external hard drive, but that's a tremendously expensive way of getting additional storage for your iMac or Macbook and doesn't use the mini in any active sense. You can't use the iMac or Macbook's display with the mini because there's no way to get video IN to either of those systems. You can, however, use remote control software to basically run the mini from either of those two systems, but in all honesty it's only worth doing that if you have an actual purpose in mind for it to be used that way.
So it really comes down to what is it you want to do with your mini? If there isn't anything particular you have in mind, then your best option may simply be to sell it. Minis seem to hold their value pretty well, so unless you have a purpose for it in mind, or simply want to keep it 'just in case', selling it would be a good option.
