Can I install a 2.5'' Seagate HHD in iMac?

While you can use a 25" drive, you will need a 2.5>3.5" adapter to get the drive to fit. You will need this adapter to install the hard drive.

If you really want to use Seagate here's the drive you want. This is from 2015, so it is likely discontinued -- I'm including it as a reference.

iMac Intel 20" EMC 2133 and 2210 Hard Drive Replacement

Okay, now for the revised answer from a new era.

STAY AWAY FROM SEAGATE. Ever since the 7200.11/7.01 drives, they have had nothing but issues with reliability! I also gave them a chance with the SSHD drives -- no real gains, and it had issues at 10k hours. The only thing Seagate is good for is target practice these days. Your best bet is to get a WD Black drive and a optical drive caddy for the SSD.

To swap the DVD drive, you will need one of these, and an external USB drive. For SSDs, look at Samsung and SanDisk.

Without specifics on this, I can only guess what computer you own. As such I am not linking guides. You need to find them.