Yes, you do need to be careful about fitting mag wheels. I'm not sure whether your question is whether race wheels will fit a road bike? In that case the answer is most often no - they may be 16.5 inches diameter or need forks and yokes that have a wider track or brakes with a different offset as I understand it (although someone who really knows their race bikes might be along in a minute).
If your question is about 'are there any road bikes that roadsized mag wheels are not suitable for' the answer is yes...you wouldn't want to fit mag wheels on a Bip because they're not designed to take the weight of two-up riding - although obviously OK if you've got a single seat conversion.
I'm dubious about secondhand mags anyway. Magnesium alloy doesn't like water very much and has an alarming tendency to turn back into crumbly white powder, so manufacturers use a special chromate primer and then paint to try and keep corrosion at bay, but you have to keep right on top of any dings or scratches on your wheel otherwise corrosion can start to work it's way under the finish. Most manufacturers advise getting mag wheels x-rayed for corrosion or stress fractures once a year and advise you to get them refinished if there are significant scratches (since these are not just cosmetic). Companies like Dymag will refinish mag wheels, for a price, but my understanding is that if they find any defects when they x-ray them, they'll just cut them up no questions asked rather than take a potential law suit when your wheel crumbles to magnesium oxide beneath you. | 