Let’s say I have a mesh for a table top and I want to add legs - would I add cylinders for the legs in edit mode or object mode? Would it be bad practice to have a table made up of two different objects if it’s not being animated? I’m new and I know this must be a very simple thing, but I’ve been very confused about it!
It is not that important, do what you need. If necessary, you can always join Objects with CTRL+J (in Object Mode: select the Objects you want to be one and press CTRL+J) or separate pieces of one Object into several Objects (in Edit Mode: press P > separate by Loose Parts or by Selection or even by Material (I've never used that last one yet...).
One time when you would want separate Objects would be for instance when different parts use different Modifiers; like with your table example: when the table top is basically a Cube with a Bevel and the legs have a profile, you could have something like this:
And on the other hand when you have a complex piece with many different parts, you might not want to have every screw and washer and.. as a separate Object, if only because of organization (every Object has a unique name and having a long list of Screw.001, Screw.002, etc is not optimal).
But like I said: it's not important, because you can always 'change your mind'!