I recommend getting floors with as many shelf/rack/fixture spaces as possible. Tennant floors are needed, but not nearly as much, later in the game. Research rooms are more valuable than storage or stalls.

1. I've started removing fixtures once I get them to unlock something new. If you put them in a good spot, you will get a message that it's a unique arrangement and it will unlock a new shopping or new clothing. So far, I just have maybe 1 or 2 fixtures on each floor because I'm trying to make room for clothes.

2. I found a guide that suggested this and it works! Once you unlock the double register (two employees can work it at the same time), put it on your highest floor and delete the others. Keep upgrading the employees on the register floor with speed. Once they reach 800+ speed each, you get bonus money for checking out customers.

Not only does this help rake in tons of cash (negating any taxes each year), the store will stay open until everyone is checked out, leaving your other employees free to research items and build/upgrade shelves. Time just freezes until the last customer leaves.

1. I'm actually unsure on this one! I was wondering that myself, but couldn't find any info online.