Is it permissible to address male colleagues in studies simply by their first name (e.g., "Ya Fulan") instead of "Ya Ustadh Fulan" in cases of necessity, and is this considered disliked (makruh) or a form of excessive familiarity?

There is no harm in dealing with a non-mahram and addressing them by their name or an appropriate title when there is a need, provided that there is no fear of temptation (fitna), along with the necessity of caution in conversation between the sexes to guard against temptation. Jurists have emphasized strictness in conversing with a young non-mahram woman.