How to increase speaker max input power
You can use it with the 1.5 W amplifier directly provided you only put out 0.5 W. Use your ears to determine when the speaker is beginning to distort badly. i.e., Use the volume control sensibly.

One cause of speaker distortion is excessive cone excursion. If over-driven the speaker is pushed out to the point where it is restrained by the suspension or is pulled in to the point where it crashes with the magnet assembly. These are easy to hear.

The other cause of damage is exceeding the heat dissipation ability of the speaker coil. In your case we can calculate from the power equation \$ P = \frac {V^2}{R} \$ that \$ V_{MAX} = \sqrt {{P_{MAX}}{R}} = \sqrt {{0.5}\times{8}} = \sqrt 4 = 2 \ \mathrm V_{RMS} \$. You could monitor this with an AC meter (but most will be calibrated for 50 Hz sinewave so bear that in mind while testing).


  Can I safely use this with the speaker by adding about 16 ohms of resistance in series with the speaker?


You could or you could put a potential divider on the amplifier input. The problem is that you won't be able to turn up a quiet passage or signal.

I'd go with direct connection and use my ears and common sense.