Can you get more than one of the same floors

You will see a severe decrease in demand for all non-food businesses (which applies to ALL businesses of that type, not just the extra ones) when you fail to keep businesses even. The demand % you see directly relates to the amount of people that will visit each floor of the type. At 100% demand, each floor gets 10 bitizens (purchases) per minute, each 10% demand drops, you lose another bitzen/sale per minute. The drop in sales will be substantial.

From the Wiki:

demand = total commercial floors / ( 5 × total category floors )
A floor of a lower demand category will draw less customer traffic than one of a higher category. For this reason, it is beneficial for the player to keep demand on all floor types high by building from each category evenly. The following table illustrates the approximate impact of various demand levels.

Thanks to demand, it's better to have food floors you just don't stock than to not have them at all, so just buy some food floors, you don't even have to stock/employ them. You can spend a tower Buck per floor to shove all your food floors together and ignore them as a set if you wish.

If you don't put employees in your food floors, you can save some bitizens for your other floors, thus needing less residential floors per commercial floor. Residential floors don't affect demand.

If you switch to/select a different mission, it will cancel the mission you are currently doing.

Other than that, there is no way to just cancel the mission and not start another (other than finishing it).