There's nothing wrong with use of informal. short forms of names of the clergy. Here's a few I can think of off hand. I knew one priest whose formal name was "Theologos," but we called him "Fr. Theo." Another priest I know, his formal name is "Demetrios," but his parishioners, largely, call him "Fr. Jim." On the flip side of this issue, I saw a young priest who I know addressed by a Parish Council member as "Fr. Mike," but the priest politely advised him, "I don't like being addressed as 'Fr. Mike;' I prefer 'Fr. Michael.' I didn't mind 'Mike' as a layman, but I don't like it as a priest." Another young man who I saw growing up was known as "Deno," but at his ordination, Metropolitan Alexios of Atlanta told him in his episcopal address, "You are no longer "Deno," you now are "Fr. Constantine." And yet, Bishop Gerasimos of Abydos of Blessed Memory, when he was a professor at Holy Cross and a priest, was known by the seminarians who had the greatest respect for him, as "Fr. Jerry." And Metropolitan Maximos, Formerly of Pittsburgh, was commonly referred to as "Bishop Max." Those who referred to him so, were most respectful of him and his ministry, and "Bishop Max" was an affectionate reference. While I don't ascribe as such, I notice in "internatz" parlance, on "facebook," even some of his priests refer to Metropolitan Savas of Pittsburgh, as "YE," i.e. "Your Eminence."
But importantly, in formal situations, such as the church's letter-head, all these clergy are referred to by their formal names. My parish priest's legal name is "Costas," and we refer to him as "Fr. Costas," but he communes himself as, "To me, Constantine, the unworthy priest..."
I do think, however, we should follow the preference of the clergyman himself. It's not for us to give them informal names--that would be disrespectful.
This topic reminds of a nun we had in our area. Among the priests who conducted serves at her monastery chapel, were a "Fr. Costas" and a "Fr. Dean," but she referred to them both as "Fr. Constandinos." I never knew to whom she was referring.
Especially in the Western cultures, our formal names, the names of Eastern Saints, are foreign to the Western Culture and with so much of our Holy Orthodoxy being intimidating to Westerners, including our own Western born faithful, why not use a name that is commonly used in the culture and not one that impedes approachability?