Were Pietro Mascagni and Ambroise Thomas both best known for their contrabution to theatre? 
yes
Ambroise Thomas
Charles Louis Ambroise Thomas (5 August 1811 – 12 February 1896) was a French composer, best known for his operas "Mignon" (1866) and "Hamlet" (1868, after Shakespeare) and as Director of the Conservatoire de Paris from 1871 till his death.
Pietro Mascagni
Pietro Antonio Stefano Mascagni (] ; 7 December 1863 – 2 August 1945) was an Italian composer most noted for his operas. His 1890 masterpiece "Cavalleria Rusticana" caused one of the greatest sensations in opera history and single-handedly ushered in the "Verismo" movement in Italian dramatic music.
 While it was often held that Mascagni, like Leoncavallo, was a "one-opera man" who could never repeat his first success, "L'amico Fritz" and "Iris" have remained in the repertoire in Europe (especially Italy) since their premieres.
 Mascagni said that at one point, "Iris" was performed in Italy more often than "Cavalleria" (cf. Stivender).
Signe Amundsen
Signe Amundsen Finsland (9 June 1899 - 13 May 1987) was a Norwegian operatic soprano.
 She studied singing in her native country with the soprano Mimi Hviid before making her professional debut at the Gamle Logen in Oslo in 1920.
 She then pursued further voice studies in Rome with Rosina Storchia and made her Italian debut at the Teatro dell'Opera di Roma in 1925 as Norina in Gaetano Donizetti's "Don Pasquale".
 She was heard at that house later that year as Rosina in Gioachino Rossini's "The Barber of Seville".
 She was then heard in the title role of Giuseppe Verdi's "Aida" at La Scala, and during her time in Milan studied singing further with the conductor Antonio Votto.
 She was soon heard at the Opéra-Comique in Paris as Santuzza in Pietro Mascagni's "Cavalleria rusticana".
 After further engagements in the French provinces, she became a member of the National Theatre, Oslo where she remained active for the rest of her career.
 As an educator, she was the singing teacher of Eva Gustavson. Valery Kritskov
Valery Kritskov is a Russian conductor who used to take conducting lessons at the Moscow Institute of Culture which were taught by Kirill Tikhonov.
 He graduated from there in 1988 and then worked in Moscow-based Helikon Opera till he got employed with Novaya Opera in 2002.
 While there, he conducted many operas including Nikolai Rimsky-Korsakov's "The Snow Maiden" and "The Tsar’s Bride" as well as Anton Rubinstein's "The Demon" and Tchaikovsky's "The Maid of Orleans".
 He also conducted works by the Italian composers such as Pietro Mascagni's "Cavalleria rusticana" and Ruggero Leoncavallo's "Pagliacci" and German such as Richard Wagner's "Lohengrin" and Strauss' "Die Fledermaus" as well as a concert dedicated to Vincenzo Bellini.
 Besides operas, he is also known for his conducting of the Russian ballet based on works by Tchaikovsky and Sergei Prokofiev as well as German and Austrian ballet composers such as Ludwig Minkus and Charles Gounod.
 Later on, he became a conductor of the Coppélia ballet which was based by Léo Delibes work and was produced by Imperial Russian Ballet.
 Currently he has two CD recordings called "Chorus of the Novaya Opera Theatre of Moscow" and the "Soloists of the Novaya Opera Theatre of Moscow". Giovanni Targioni-Tozzetti
Giovanni Targioni-Tozzetti (17 March 186330 May 1934) was an Italian librettist, best known for his friendship and collaboration with the composer Pietro Mascagni.
 Most of his libretti were written in collaboration with Guido Menasci. Marie Cabel
Marie Cabel (31 January 1827 – 23 May 1885) was a Belgian coloratura soprano.
 She is probably best remembered for having created the role of Philine in Ambroise Thomas's opera "Mignon". Immortal Melodies
Immortal Melodies (Italian: "Melodie immortali" , also known as Mascagni) is a 1952 Italian musical-biographical drama film directed by Giacomo Gentilomo and starring Pierre Cressoy and Carla Del Poggio.
 It is based on real life events of classical composer Pietro Mascagni. Bernhard Bötel
Bernhard Bötel (1883–1953) was a German operatic tenor and actor who had an active career in Germany and Austria during the first half of the 20th century.
 He made recordings for several record labels during the early years of the recording industry, including His Master's Voice, Odeon Records, Polydor Records, Tri-Ergon, and Vox Records.
 On the stage he sang a variety of roles in operas and operettas from leading parts to comprimario roles.
 His stage repertoire included Belmonte in Wolfgang Amadeus Mozart's "Die Entführung aus dem Serail", Chapelou in Adolphe Adam's "Le postillon de Lonjumeau", Count Almaviva in Gioachino Rossini's "The Barber of Seville", Daniel in Franz Lehár's "The Merry Widow", the Duke of Mantua in Giuseppe Verdi's "Rigoletto", Gabriel von Eisenstein in Johann Strauss II's "Die Fledermaus", Indigo in Strauss' "Indigo und die vierzig Räuber", Jeník in Bedřich Smetana's "The Bartered Bride", Paolino in Domenico Cimarosa's "Il matrimonio segreto", Pâris in Jacques Offenbach's "La belle Hélène", Pietro in Franz von Suppé's "Boccaccio", and Wilhelm Meister in Ambroise Thomas' "Mignon". Le roman d'Elvire
Le roman d'Elvire is an opéra comique in three acts composed by Ambroise Thomas to a libretto by Alexandre Dumas ("père") and Adolphe de Leuven.
 Also performed under title Fantaisie de marquise, it was premiered on 4 February 1860 by the Théâtre Impérial de l'Opéra-Comique at the second Salle Favart theatre.
 Set in 17th-century Palermo, the opera blends elements of realism and fairy tale in its story of a young libertine who discovers that the wealthy old woman he thought he had married is actually the young and beautiful one whom he had jilted. Carlo Lombardo
Carlo Lombardo dei Baroni Lombardo di San Chirico (Naples, 28 November 1869 - Milan, 19 December 1959) known also under the composer-pseudonyms Léon Bard, Leo Bard, Leblanc and M. Fernandez, was an Italian operetta impresario, comedian, librettist, publisher and "composer" of pasticcio productions of other composers' music.
 He is regarded in Italy as the father of the late 19th and early 20th Century revival in Italian operetta.
 He was responsible, in a somewhat debatable manner, for getting Pietro Mascagni to write the operetta Sì.
 His brother was Costantino Lombardo, Italian conductor and composer.
 His publishing house, Lombardo Editore, continues to publish sheet music for operettas.