Since both affect serotonin and there is no clear guidance about at what dosing such mixing can lead to problems, I should avoid using this combination until checking with the doc.

This class of antidepressants has not been notably helpful with CH, if this is the primary purpose in using the Remeron. IF you must stay on it, Verapamil would be a better choice to reduce/prevent attacks and can be used with the R.

my guess is that you take it before bedtime because of the side effect of tiredness when you take it?  I used to work in a mental health clinc and I recall one of the side effects   that patients complained of was fatigue hence the bedtime dose. It is a pretty major anti depressant!  Not your typical -      

With that said-  I would check with a doctor about taking the melatonin with it as others have stated.  If remeron causes you sleepiness, wait till you take the melatonin-