Where are we shown Rose's surname?
Rosé's surname is mentioned in the 2004 FMA game Fullmetal Alchemist: Stray Rondo released by Bandai.

This is clear from this video where the aforementioned game is played on an emulator.

If you go to 2:49 you can see Rosé introducing herself to the Elric brothers as: Roze Tōmasu (in Japanese writing, katakana is used for writing non-Japanese names, ロゼ・トーマス) which is then adapted to English as Rosé (not like the flower rose) Thomas.

This surname appears neither in the manga nor in the anime. It is common however for wikis to adopt such information from officially released games.

Here is the aforementioned screenshot: