1st, 2nd, 3rd conditional and beyonndddd!!

"If he's going to come, I'd ask him."

That's the only one that sounds off. You have a future conditional in the first phrase. Then, you have a past or non-temporal conditional in the second phrase.

I'd say: If he's going to come, I'll ask him. OR
If he's was going to come, I'd ask him.



"If he might come, I'd ask him."

This one also seems to benefit from I'll as opposed to I'd. The first phrase still implies future, although not as clearly.

Still, I can think of an instance where the sentence works just as you wrote it. However, in that instance, the better phrase would be, "If he would come, I'd ask him."

Another meaning is conveyed by this sentence:
"If he might have come, I'd have asked him."