THE 1991-92 Houston Grand Opera (HGO) season is another fascinating mix of the new with the old. The 11-production schedule began with a stunning production of Dvorak's "Rusalka," and was followed by Puccini's "La Boheme."The most noteworthy new production is Robert Wilson's staging of Wagner's "Parsifal" with Houston Symphony Orchestra music director Christoph Eschenbach conducting. The new work will be Robert Moran's "Desert of Roses," a variation of the beauty-and-the-beast legend, which the HGO commissioned. It will alternate with Gretry's 1771 opera mire et Azor" (renamed in Houston "Beauty and the Beast"). Also on the docket, Boito's "Mefitofele" (with Samuel Ramey), Giordano's "Andrea Chenier" (with Eva Marton and Giuseppe Giacomini), Verdi's "Il Trovatore," Sondheim's "A Little Night Music," Berlin's "Annie Get Your Gun," and Herbert's "Babes in Toyland." "Rusalka" would have made any house proud. The story of a water sprite who seeks human form for love of a prince is a designer's challenge, since the first and third acts take place in and around a lake. Gunther Schneider-Siemssen's designs and lighting were magical. Water effects, particularly when Rusalka walks on the lake, were superb. In the title role, Renee Fleming proved yet again that she is well on the road to operatic superstardom. She has a stage presence that matches her radiant and effortless soprano, and she met all the various hurdles of this vocally tricky and taxing part with astonishing ease. Her faithless yet adoring Prince was Neil Rosenshein in a vividly compatible role: His tenor gleamed handsomely in all the right phrases. Sue Patchell made much of the often thankless role of the Foreign Princess; Heidi Grant's sopr ano took on rich mezzo hues as the sorceress Jezibaba, and Richard Paul Fink revealed a robust baritone in his performance of the Water Gnome. Julius Rudel conducted the score with considerable panache and, more crucially, affection. The "Boheme" was, despite certain flaws, an enjoyable evening, carried by the vigor of the predominantly young cast. The main problem lay with Keith Warner's ill-conceived direction which at too many junctures fought Puccini's music. As for the singing, only baritone Gaetan Laperriere, the Marcello, offered a consistently excellent vocal and dramatic performance. Lucia Mazzaria's Mimi was colorless and unaffecting. Marcello Giordani is a young, inconsistent tenor of tremendous potential and a very appeal ing stage presence. Who could not be disarmed by his Rodolfo? George Hogan was the solid Colline, and Jan Grissom's Musetta would have been more winning with more flattering stage direction. All were carefully supported by conductor John DeMain, who gave a particularly fine account of this ever-popular score.