Not quite. "Welke" is used for de-words and plurals. "welk" is used for singular het-words, like "het meisje".
Note that these are the standard adjective rules. The adjective "welk" acts as if there was an indefinite article ("een"). See that link for a list of other adjectives like that.
Thank you! So this quite the same as adjectives. Is it the same with "enkel" and "enkele" etc?
Yes, "enkele" follows the same adjective rules, but note that it does not replace the article, like "welke" does.
Een enkele boterham - A single sandwich
is there a list of de/het words somewhere that I can study ? Because I feel like it will never stay in my brain otherwise
Welk meisje is correct. Welke meisje isn't, because meisje is neuter. All dimunitives are neuter.