"The best ISO is usually higher"
>this is wrong! For minimum noise the always use the lowest iso setting. Low iso (reduced light sensitivity) and gives you the lowest noise to but requires longer exposure times to get the proper exposure. Hi iso will give you Short exposure times but a lot more noise.
This is sort of correct, in that you should always use the lowest ISO for the desired aperture and shutter speed. If you later brighten the picture using software, then you would have been better off noise-wise using a higher ISO (provided greater shutter speed or aperature weren't possible). That is only true up to an ISO which varies by camera at which point brightening in software is preferable (ISO 1600 for mine).
Addtionally, longer exposures become noisier on a digital camera. Eventually there is a tipping point between using a higher ISO and using a shorter exposure length. I tend to assume 60s for my present camera, but that is mostly because that is the start of Bulb mode. I have tried 4, 8, and 16-minute exposures at ISO200 and they are clearly too noisy, but I don't know exactly where the tipping point is. I expect the tipping point will also vary with outside temparature (see below). In summary, minimizing noise with digital cameras is a trade off between minimizing ISO and minimizing exposure length, and is camera-specific. I would guess for a Canon T2i the 60s rule would also be a good way to determine when to raise the ISO as far as 800 or 1600, as with my E-M5.
When using exposure stacking software like startrails.exe, random noise will cancel out in favor of the brighter pixels, but hot pixels will accrue. Hot pixels are a product of longer exposures and higher temperature, thus shorter exposures and colder temperatures accrue less of them. It isn't possible to do black frame subtraction after every shot because the star trails become a dashed line; therefore, short(ish) shutter speeds, high(er) ISO's, and stacking are the way to get the least noise in a star trail image. The exact point for best image quality varies by camera.
"There will be less noise in colder temperatures."
>for all practical purposes you will see no significant difference until you get down to temperatures below -50F. At normal temperatures ranges you will see no significant difference.
This is wrong! There is a clear difference between shooting at 0 deg F and 60 deg F. Colder temperatures are especially beneficial for longer exposures. A cold temperature goes a long way toward reducing the noise-generating heat created by the sensor during the exposure. Have you tried? Alternatively, the internet is a good source of information.
If your pro gear is relevant to this discussion, the only possible way is because your camera suffers less read noise and hot pixels than the cheapo models. This makes your advice to the OP worse, rather than better.