Ah, in that case maybe I shouldn't have them all set to "Balanced" then.Its fine, you can just leave them in basic trucks, it doesn't really make any difference. What matters is the driver's skill. General advice is focus on long distance driving skill first for them along with the perhaps an ADR or two appropriate for their location.
At first they won't make you much money, so its mainly about letting them build up experience. When they get some experience and skill points invested, they start bringing home the bacon, regardless of what they are driving.
Later on, which you are rolling in cash, you might splash out on better rigs for them.