What is considered low total testosterone for a 23 year old male? Would 400-420 warrant replacement therapy, or HCG/Clomid?

Testosterone. No. This level is perfectly normal for a man of your age.
Low (T)estosterone. Testosterone levels vary according to the time of day with higher levels in the morning hours. An accurate assessment is best taken in the early morning. Normal reference ranges vary among labs but usually greater than 200ng/dl is normal. Of course low normal levels with symptoms of low T may warrant replacement. Lab studies as they relate to clinical signs and symptoms may lead to treatment.
Not Low. 400 is well within the normal range. You may want to consider speaking to your doctor about other causes of your fatigue if you are having it.
Hypogonadal. low testosterone is frequently over diagnosed due to inappropriate testing. First, do you have impotency? Second, do you have low libido? Third, do you lack MORNING erections? If you answer no to above testosterone should NOT BE MEASURED. It should not be sought as an issue for tired or fatigue. The appropriate test is free testosterone LH and FSH to evaluate for low testosterone.