If you find a partner after a tanda has started, and you want to enter the line, don't just dart into the ronda, cutting someone off..... Gentlemen, get into an embrace with the lady off the floor, look at the guy that you would like to step in front of and make eye contact. Once eye contact is made, nod towards the floor to indicate that you would like to enter the ronda.
I think this applies more where there is limited access onto the dance floor or the floor is very busy otherwise I think you should either look for or wait wait for an appropriate space to become availabe because you should try not to disturb anybody even to the extent you have suggested.