Which type of high blood pressure medication would be best to combine with a low dosage of beta blocker?

Any. The low dosage of beta blocker would not especially influence my choice of a second agent except for the fact that medications like verapamil or Diltiazem also slow the pulse as does the beta blocker. So one would want to make sure that the beta blocker has not significantly lowered the pulse.
HCTZ (hydrochlorothiazide) Without knowing more about you, especially your renal function, allergies, and other medical conditions, it's hard to make a good recommendation for you. But in general, a mild diuretic such as hydrochlorothiazide (hctz) is usually a good choice - either by itself or in combination with a beta blocker.

Related Questions

What type of high blood pressure medication would be best to combine with beta blocker?

Just about any. We often combine beta blockers w/mild diuretics, ace inhibitors, arb, ccb, etc. Many patients need 3 or more BP lowering medications to achieve their goal bp. It's just a matter of choosing a drug from each class, looking for secondary gain if possible, while avoiding side effects. Diuretics are good if you have swollen feet. Ace inhibitors & arbs protect heart in CHF & kidneys in diabetcs.