Yes! You can safely mix Tru-Tone LED bulbs with traditional incandescents on the same light string, so long as the total wattage of the strand doesn't exceed the maximum rating for the cord in question.
For example, our light strings can safely handle up to 360 watts – using only our C9 LED lights (0.6 watts each), that means you can safely power up to 600 bulbs on one circuit.
Incandescent C9 bulbs tend to use around 7 watts each, so doing the math (360 watts divided by 7 watts each) you can only power about 50 incandescents on that same string safely.