Can a soldier wear IHWCU top with regular OCP ACU pants?

I wouldn’t mix and match. You can tell the difference. I also have 4 sets of ACUs. I have them all labeled 1-4. The trousers and blouses are all worn together and washed together so that they fade the same. Old trick an SFC I worked with told me to do.

Attention to detail is a good thing.

The regulation for the IHWCU doesn't exist yet, the wear guidance comes from ALARACT 027/2019. The ALARACT doesn't mention this specifically, however it does say:

AFTER 1 JUNE 2019, SOLDIERS CAN PURCHASE THE IHWCU FROM MILITARY CLOTHING SALES STORES FOR WEAR IN LIEU OF THE OPERATIONAL CAMOUFLAGE PATTERN (OCP) ARMY COMBAT UNIFORM.

One uniform is worn in lieu of the other uniform, it does not address items at a component level, my interpretation is that it's all or nothing.

Don't wear mismatched uniform components. It speaks to your attention to detail. Trust me people notice.

Make sure you match the same blouse with the same trousers and the same cover (even within the same pattern). That should make sure that you even have the same amount of wear/laundry fade.