THE new Collegio Strumentale Italiano is a first for Italy, a group of young Italian colleagues dedicated to performing pre-Romantic music on authentic instruments: recorders, Baroque trumpets, oboes, and bassoons; harpsichord; positive organ; and strings. ``Tafelmusik'' by Telemann - a bit of light ``dinner music'' - opened their recent concert in Modena, the final program of their first season. Conducted by Gustav Leonhardt, an international authority on early-music performance, the program included an inspiring performance of Bach's ``Easter Oratorio'' and ``When Saul Was King,'' a trite anthem by Bononcini of Modena, who was Handel's great rival in 18th-century London. Florence, Italy FLORENCE'S annual May music festival, the Maggio Musicale Fiorentino, opened its 50th season this year with a lavish, spectacular production of Berlioz's seldom-heard opera, ``Benvenuto Cellini.'' It was a musically rewarding performance, sensitively conducted by Vladimir Fedoseev, with tenor Chris Merrit as an outstanding Benvenuto Cellini. Although 1987 marks the golden anniversary of the Maggio, there seems to be no special ``theme'' or ``big-name'' appearance to mark the occasion. Rather, it looks to be a solid season of mixed operas, concerts, and recitals that ends July 1-4 with Purcell's ``The Fairy Queen.'' Ferrara, Italy WHEN young Niels Nielsen was a piano student at the Conservatory in Bologna, his father, Italian composer Riccardo Nielsen, made a gift of a pair of two-piano works that they could play together. So it was only natural for the son, now a fledgling conductor, to direct the Philharmonic Orchestra of the Bologna Opera in a memorial concert dedicated to Riccardo Nielsen. A rather lifeless ``orchestration'' for string orchestra of Bach's ``Chaconne'' for unaccompanied violin preceded the main work, ``Fasce sonore (6 + 5).'' (The numbers refer to six violins plus five lower stringed instruments.) Complex in sound, dodecaphonic in texture, ``Fasce sonore'' proved a fascinating bit of contemporary serial writing.