Does that make Ravan the grandson of Tadka? Do the scriptures mention Sund and Sumali to be the same person?
As per Valmiki Ramayana, NO:
Tataka was daughter of a yaksha named Suketu. She was married to Jambha's son Sunda. Tataka and her son Mareecha attained demon-hood due to curse of sage Agastya. ~Valmiki Ramayana: Bala Kanda: Sarga 25
On other hand Malyavan, Sumali, and Mali were sons of a demon named Sukesa. Their wives were Sundari, Ketumati, Gandharvi who were daughters of Narmada (a Gandharvi). And none of the children of Malyavanta, Sumali, and Mali was named Mareecha.
Malyavan's children were Vajramushti, Virupaksha, Durmukha, Suptaghna, Yajnakopa, Matta, Unmatta and Anala. Sumali's children were Prahasta, Kampana, Vikata, Kalikamukha, Dhumraksha, Danda, Suparswa, Sanhradi, Praghasa, Bhasakarna, Pushpotkata, Kaikasi and Kumbhinasi. Mali's children were Anala, Nila, and Sampati. ~Valmiki Ramayana: Uttar Kanda: Sarga 5