If you mix lamps and fit HID's, the light patterns are not correct when tested by an MOT test centre.Thus potentially blinding the poor oncoming motorist.
Xenon headlamps are the ones to use .If set on the lower side on height adjustment, in case your carrying a heavy load will insure you don't blind anyone as the light direction would be aiming high if your carrying 4 or 5 people or a boot full.
You must also insure if you have swopped your halogens for xenons, that you also remove the levelling switch from the halogens from the dashboard as that can be seen as "inoperative"and a potential failure. Easy replaced with a new switch fitted from cars fitted from a factory spec.
This is only if you don't want the extra expense of auto levellers /washers of nearly £400.