The Trinket doesn't have a hardware Serial port, so you have to use SoftwareSerial, but that should be able to transmit at 115200 baud (which the Pixie LED wants).
WRT temperature, the calculations combine differential equations from thermodynamics and fluid dynamics, and you have to guess at a lot of the constants. In practice, everyone does it empirically and publishes the results as "don't use more than this much power." The datasheet for the LED specs it as having 3W maximum power dissipation, a peak continuous current of 350mA, and a peak pulsed current of 1A for 1ms on no more than a 10% duty cycle:
https://www.adafruit.com/images/product ... RGB-Y2.pdfYou'd get some small advantage from adding a fan, but that's largely irrelevant to LEDs. The unwanted effects of heat happen right at the junction where it's generated, mostly producing lattice defects in the silicon. That increases the resistance through the junction, which generates more heat, which causes more defects, etc. It's an exponential process that starts from a small constant, so it builds up slowly then gets big quickly. The effects are measured as a drop in luminous output, with "time to 50% of initial value" being the figure of merit.
Most LEDs are designed with a half-life between 5000 and 10,000 hours at nominal current, and running at a different current has roughly an x^4 effect on half life. Running at twice the nominal current shortens the half life by a factor of about 16. Running at half the nominal current extends the half life by a factor of about 16.