The 15 hours has been in JAR-FCL 1.330 since JAR-FCL was first introduced. There were anomalies in that it appears in JAR FCL 1.330 privileges of an FI, but does not appear in JAR-FCL1.325 privileges of a FI(R).
Type in this case is a Type as defined in JAR-FCL1.220. Cessna 172 and PA28 are both in the SEP Class and do not constitute different "types" so you do not need 15 hours on each.
The CAA recommend that you have a minimum of 5 hours experience on a different aeroplane within the class before instructing on it.