1. With the bike standing next to you on the ground, lock the front brake
and push the bike back and forth. Do you feel any play in the fork/headset ?
There should be none/zero/nada.

2.Take your old fork and hold it so that the tube lines up with your frame's
head tube, alongside the newly installed fork..... get someone to measure
how far the new fork's ends sit relative to the old fork's ends (fore and
aft in relation to the BB. This is roughly how much you've either
increased or decreased the rake. You can get away with some decrease,
but past a certain point, the twitchy feeling you describe becomes more
pronounced. ( For the record, rake is actually defined as the difference
between point of tire contact patch and where imaginary line parallel to the
head tube would intersect the ground.)

3.In my experience it is never a good idea to mix headset parts....too many
variables and ways to **** up.