Can a 3.5 hard drive be installed in a 2.5 SSD slot?

If your trying to build a PC and the case has 7 expansion PCI slots with 2 X 3.5 HDD and 2 X 2.5 SSD space can you insert another 2 hard drives into the 2.5 SSD if theres no space? Can you also insert the hard drives into the expansion slots instead?
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