AC Plugs, different types of plugs/receptacles
One strand has "like sized" prongs/slots (I'll call this Type A) and the other size has unlike sized prongs/slots (Type B). 


Your 'Type B' is properly known as a polarized plug.

It is unsafe to connect an unpolarized plug to a polarized outlet. This is because the purpose of polarization is to reduce the chance of your touching dangerous line voltage such as on the threads of an empty bulb socket; North American power has a “hot” and “neutral” wire, and the “neutral” is generally closer (but not necessarily equal) to earth potential (voltage) than the “hot”, so it is less dangerous (but still unwise to touch).

You should not grind down a pin to make it fit. Would it work, electrically? Yes. But it is removing the designed safety features.

(From a strictly technical perspective, it would be safe to plug in such an altered plug to a GFCI outlet or portable GFCI — GFCIs provide better protection against the same hazard than polarized plugs. But then you would still have an unsafe plug configuration around in the future.)

Use extension cords (of the type designed for outdoor lights) instead. This also has the advantage of ensuring you are not overloading the pass-through outlets with more strings than intended.