I sold a condo in Fremont last year with the same situation and the people on the first floor did not sign their approval. However, the buyer still put hardwood floors and told the people downstairs that he would put an extra thick underlayment under the hardwood floors and put some rugs around. He also told them that he did not allow friends and family to walk around in shoes inside the unit. If the people downstairs still were bothered by it, he would take out the hardwood floors. Well, he still has the hardwood floors and has not been asked to take them out. He is also a very friendly person and tries to keep them happy.
To answer your question, you are taking a risk because it will not be easy to convince the people later. They start thinking of how your flooring will affect them when they go to sell. So don't depend on it. My client was willing to take that risk.
If hardwood floors are very important to you, look for a unit on the first floor.
If I can help in any way, please call me
Help-U-Sell Achievers Realty
40083 Mission Blvd
Fremont, Ca 94539