MBA Abroad Querries - Help Needed

For getting admission in top 20, is it necessary (or highly recommended) to have senior level or middle level management experience instead of junior level experience? I have few questions regarding the recommendation. I would like to inform my …

For getting admission in top 20, is it necessary (or highly recommended) to have senior level or middle level management experience instead of junior level experience?
I have few questions regarding the recommendation. I would like to inform my referees so that they write the stuff in a better fashion.
The generic dimensions, e.g. leadership, team work, integrity etc. are asked for in the standardised rec form; any specific aspects one has to pay more attention to ? How long would a very good rec be ?