Why not 'ny og pen'?
Because 'hus' is a neuter noun.
'Ny og pen' would be correct if you were describing a masculine or feminine noun.
I thought it would be 'nye' and 'pene' because huset is the definite.
The notes don't explain this, but I picked it up from a comment elsewhere: if the adjective comes after "er" (as opposed to before the noun), it's always inflected as indefinite, even if the noun is in the definite form. So:
Det nye huset er nytt.
Why is vakkert wrong?