For example in the following sentence should the adjective 'magnus' also take 'que' to agree with the noun 'puer'?
Puella puerque magnus.
For example in the following sentence should the adjective 'magnus' also take 'que' to agree with the noun 'puer'?
Puella puerque magnus.
No. Treat it not like a form of the noun, but a separate word that gets attached at the end. It functions just like et except it goes after, not before the word.
See this thread for more information.
Note that -que gets attached to the next word it conjoins. You could have also written puella magnusque puer and it would have been correct still. Thus in Cato we read:
de omnibus agris optimoque loco...
Concerning all the fields and the best place...