Would "το κεφτεδάκι μου" also be correct?
it should be, it's the same thing
is κεφτές a word originated from Turkish?
Yes, from the word ''köfte'' which in turn derives from persian.
is there a rule of thumb on what gender foreign words are? For example, sandwich and cheesecake are neuter, and keftes is masculine. If i have to decide on a gender for a foreign word, what's a good way to do it? (Other than memorizing it)
Usually loanwords are almost always assigned to the neuter '' το βίντεο'' '' το ταξί'' ''το αλκοόλ'' if they denote inanimated objects. Exceptions still exist though..