Hi, after consulting the reference I would say yes! However, you normally have 2 RAM slots, so it would be better to get a 2x16GB kit in order to get the most out of the memory (Dual Channel etc...)
Yes.
In principle, you should install ram bars in pairs, otherwise you will not be able to use the full function, i.e. dual channel. In everyday life, however, you will probably not notice a big difference when doing simple work on the Mac. I would therefore recommend that you take a 2x16 GB kit and install it in addition to the existing memory. The standard 3.0 GHZ iMac takes a maximum of 32 GB RAM, all others with faster processors 64 GB.