No, this is not a rule that applies to all ντ = νδ. There are some words that have two forms of writing such as the numbers εφτά = επτά, οχτώ = οκτώ, εννιά = εννέα.
Hope it helps.
I think I have heard adio and andio. I think adio is good for those who realise it is a foreign word and andio who do not realise that. Am I correct?
In general Greeks don't differenciate between nd/d (also mb/b) sounds. They are both written as "ντ" and pronounced as "d" in the beginning of a word and either as "d" or "nd" in the middle. It doesn't matter if it's a foreign word, we're not supposed to adapt to pronunciation rules of foreign languages. The word "μάντης" (=oracle) , which is greek is pronounced either as "mandis" or as "madis" by us natives.
Good call out by YPSILONZ! I also wanted to say here that same thing happens with double letters. Greeks don't differentiate pronunciation of double letters. For instance άρρωστος can be simply pronounced as arostos or arrostos - same for άλλος = allos = alos. Double letters not always mean a different pronunciation as in other languages like Italian or Spanish.
Άνδρες. It is not wrong, specially when we use the term in a scientific meaning, Biology. Also correct are the term ανδρισμός, ανδρικός etc. This τ, when used, gives a tougher meaning to the word, to express power etc. In Ancient Greek and in Katharevousa it is definitely άνδρες