Just picked up a super cheap centre speaker as a bargain, I am going to be grabbing an AV receiver and two more L&R speakers next week to match it, but was wondering if I could disconnect my current speakers from my stereo amplifier and plug the centre into the terminals for the left or right speaker so I can get an idea of how it sounds, or is this dangerous?
Yes you can plug in the left OR right terminals into the speaker to test it. It'll play back the sound for the channel you plug into it. The only risk of damage is if it's designed for a different impedance than your current speakers, and you have the volume set too high.
But as stated above, to get the center speaker to actually produce any sound in regular use from just stereo input, you need a 5.1 or 7.1 receiver which not only amplifies, but will convert stereo input to fill out all 5.1 or 7.1 speaker channels.
Edit: deleted 4.1 since that doesn't have a center channel either.