Are there any guidelines or regulations for seat legroom on airlines?
The short answer is no, beyond the need for an aircraft to satisfy the emergency evacuation requirements set by the FAA and other regulators. There must also be room for an approved seat cushion and seatbelts. So there is a point where a lack of legroom would impede evacuation to the extent that an aircraft couldn't pass the 90 second evacuation requirement, but there's generally no specific minimum legroom requirement.

The main exception is in the UK, which does have a form of minimum requirement (according to that link, the UK is the only country with one). The requirement is given as a different form of measurement from how airline seat pitch is normally defined, but the above link indicates that it basically translates to a 28-inch seat pitch. That's about as low as any airline goes, namely Spirit in the US. Most airlines draw the line at 30", with a few low cost carriers doing 29"