Mines had two anesthetics, and shes been fine. The biggest worry really is not the actual anesthetic gas (isoflurane), but the heat loss during the procedure and during recovery. Because they're so small they have a high skin area to body ratio and lose heat easily (as they're not moving around and keeping warm under GA), so hypothermia is the worry. To prevent this guinea are normally kept warm by heated pads, towels, fluffy bedding, bubble wrap.. and if they need sub cut fluids it is usually warmed up before it is given.
During recovery, pigs need to be kept warm again via heat pads etc...
At my vets they use an incubator where the temperature can be controlled.
Plus, when giving consent for general anesthetic, it is a legal obligation to make the owner aware or the risks of GA, and as guinea are small, and if they are healthy, is normally a medium risk.