DeKay,in older electrical panels the amperage designation on the circuit breaker handle, which is usually highlighted with white or red paint,can partially wear off. It would be easy to mistake 150 amp for a 15 amp.If it is a 15 amp C/B then it is probably faulty, as under normal household operating conditions it would not be large enough to supply the branch circuits throughout the house without constantly tripping.In general,the max. C/B rating is determined by the ampacity of the wire which takes into consideration conductor size, type of insulation and ambient temperature.But if it is 150 amp the wire size should be much larger than the wires attached to the 15 and 20 amp branch C/B's.In older panels anything is possible.A lot of changes could have been performed by owner, tennants,handymen etc.I would have a licenced electrician check it out.
A 15 amp recepticle is rated for both a 15 and 20 amp circuit, so you can install it on #14 or #12 AWG.A 20 amp rated recepticle can only be installed on a 20 amp circuit with #12 AWG or #10 if rated for #10 AWG. Hope this helps.