How can I determine how powerful certain herbs and spices are? Is there a list?
I doubt you could even do this for different brands of hot chilli powder (for example). Never mind trying to come up with an equivalent between things that aren't equivalent. Fresh chillies are even more variable. Strong flavours like herbs and spices interact with the other flavours in a dish, so something you find equivalent in one dish won't be right in another. Example: cumin can overpower a mild curry if you're not careful, but a large batch of my slow cooked chilli could handle a huge amount, while too much cinnamon in either wouldn't taste right, and the curry could take a decent bit of ginger while the chilli would cease to taste like chilli. 

Ginger, mustard and chilli are all hot spices, but the difference is one of kind, not just degree. You even feel the heat in different places (chilli more in the throat, mustard in the nose). Further, one person's perception of these different kinds of heat will be very different to another's - I could say that a sample of mustard is hotter than a sample of chilli sauce. You could say the opposite, and we could both be right.

The solution, to a large extent, is to test when making an unfamiliar dish or using unfamiliar ingredients. Accepting some variability is also necessary. Most herbs and many spices can be adjusted towards the end of cooking, sometimes by using a different form. Of course you need a rough idea of how much to start with, but you won't get that by replacing some amount of thyme with basil.