Can I plug any ol' speakers into a surround sound reciever?

Be careful; you should check the ratings for speakers as they can be damaged if underpowered. And by underpowered, I mean both the output wattage of the channel they’re plugged into, and the impedance rating; if mismatched (for instance, a larger speaker designed to be driven at a low impedance like 4 Ohms, plugged into an amp channel rated for 16 ohms), then the speaker may be underpowered on account of the increased resistance (impedance) on the line. The telltale symptom of this is a speaker that plays semi-loud but sounds severely distorted. You risk permanent damage to the driver units in the speaker when you do this. When in doubt check the reference manuals for both.

This depends on where you are placing the speakers. For the rear channel speakers, you can use pretty much anything you have lying around, but if their characteristics don’t match the main speakers very well, you won’t like the sound.

If you plan on adding a center channel speaker, it will need to be magnetically shielded if you’re going to place it near the set, at least, if you have a CRT set. However, you can use pretty much anything for that, too, subject to the same caveats. I used one of a set of KLH outdoor speakers I had lying around for that purpose, and it does an acceptable job.

@sndfreQ, good point about checking the impedance rating of the speaker v. the amp. A lot of surround receivers are rated for 8-ohm loads, and 4-ohm speakers can overload them even at moderate volume levels. But I haven’t seen a 16-ohm amp (or speaker) since the 1960s.