While agreeing with someone on something, can I use any of these instead of the simple "I agree with you"?
I have to agree with you.
I'll have to agree with you.
I'm going to have to agree with you.
I'd have to agree with you.
Have to X means to be required or forced to X.
So, I agree with you means you agree of your own free will. I have to agree with you means something is forcing you to agree.
The difference between this and plain "I agree" is that when you say "I have to agree", you are admitting you are wrong in light of some new information or evidence, or that you wish it was not true but can no longer deny it.
Will and going to, in 2 and 3 above, might literally mean "in the future, I will be forced to agree with you". However, it's usually and more likely meant to express deference/politeness/respect to the other party of the conversation.
Modals are often used to express deference/politeness/respect and their literal meaning is often not truly intended.
This also applies for would, in 4 above--but would can also express that "I have to agree with you" is the "then" part of an "if-then" condition. The if X could be in the same sentence, an earlier or later sentence, or implied by conversation.