Is there a particullar startup order for the different voltage rails on caravel, or can I just power them all at once?

The power-on-reset circuit responds to the 3.3V supply, so you don't want a huge delay between bringing up the 3.3V supply and the 1.8V supply, since the power-on-reset assumes that the 1.8V supply is stable by the time it triggers (which is about 50ms after the 3.3V supply reaches some mid-range value). Generally, I'd recommend bringing up both supplies at the same time and ensuring they both reach a stable value in less than 50ms. If you can't ensure that, then bring up the 1.8V supply first.