Well not sure, the Redstone-Engines used to produce 0,1 EU/t, which will only output if it reaches 1 (so every half second).
Nope, Buildcraft engines output energy "per stroke", meaning they dump whatever they have accumulated internally the moment the piston reaches the front. This is true for all Buildcraft engines, and
should be true for all Buildcraft compatible engines (though I can't prove it).
A redstone engine generates 0.05 MJ/t, regardless of piston speed, which adds up to 1 MJ per second. However, it is special in the way that it has a limit of 1 MJ output per stroke. Since it strokes much slower than once per second initially, it will start building up energy inside itself because the piston doesn't move fast enough to get rid of it. As more and more energy builds up, the engine changes color and the piston moves faster and faster. Once it hits the red stage, the piston moves faster than once per second, meaning it dumps energy faster than it can produce it, and therefore it drops down to yellow. At which point the piston slows down to less than one stroke per second, meaning it once more generates more energy than it can dump, which accelerates it to the red stage...
That is why Redstone engines flash yellow/red when reaching "optimal temperature". They hit an equilibrium exactly between those two states where the stroke length is exactly 20 ticks and therefore the output is exactly 1 MJ per second, which is exactly how much it produces internally.
Other engines aren't as hard limited. For example, a stirling engine will never move away from blue on its own, because every stroke can dump far more energy than it can produce internally during the time, even on blue. They will only change color if there's nothing to accept the energy (or not all of the energy), or other engines daisy-chain into it. Stirling engines are made for daisy-chaining.
A combustion engine doesn't even change color based on internal energy, but rather on temperature which is generated alongside energy but not dumped by the piston. That's why it needs cooling.