At this point it is really your choice which tartan you wear, I for one like to have some kind of connection to the tartan i am wearing, but many will say it does not make any difference, the important thing to alot of people is that you wear it with respect, and you might want to learn a few things about the history of the clan or tartan.

As far as a family sgian dubh I have seen sgian dubhs with clan crests in the handle if that is what you mean, I would suggest that you do a search on the forum regarding all of this I am sure you will find alot of answers.