～とも (in the sense you seem to be talking about) is a rather archaic sentence-final particle which is used for strongly asserting something that the listener may not be so sure about. That makes it similar to the far more colloquial particle よ, but it's somewhat stronger and more decisive than よ. 「いいとも」, for instance, would usually come in the context where A asked B whether it's okay to do something, and B wants to firmly reassure A that it's okay.
This とも has no relation whatsoever with ～と思う, but rather comes from combining the particles と and も. かしら, on the other hand, indeed comes from かしらん (＝かしらない), as Dave has already indicated.