Hi,
My vote for auxiliary lights on a TR6 would be rectangular. I just think those would look best on the car and are "period correct". I'd hang them below the bumper, or sit them atop it, depending upon the size and type of light and your preferences. Spot/driving lights would work best mounted a little higher. Fog lights generally work best mounted relatively low. However you do it, be careful to minimize blocking airflow to the radiator.
I do agree the round lights look "right" on the AH, and prefer them on my '62 TR4 and earlier cars.
I've used Hella 500 series on various cars over the years. These come in a choice of round or rectangular, are well made, work great and are reasonably priced.
With a pair of round lights, the one in front of the driver is often a long range driivng/spot light, while the one in front of the passenger is a fog light, and they are switched separately. The fog light might be relay connected to the low beams, while the spot is relay connected to the high beam. This is the factory set up on my Land Rover, which has both fogs and driving/spot lights.
Mostly it is just the design of lens that is mounted in the lamp housing that differentiates driving/spot from fog lamps. Sometimes the lenses were also different colors, clear for driving/spot and yellow for fog.
However using two different types of lights isn't a hard and fast rule, by any means. Different drivers used different lights. For example, the works TR4 rally cars were set up with 2, 3 or 4 auxiliary lights added to the front, plus one or two on the rear as reverse lights! Various combinations of spot and fog were used on the front, while fog were usually used for reversing lights.
In most new sets of lights you might purchase, both are the same type. And, if you chooose rectangular, the types aren't usually mixed.
By the way, it's a good idea to add additional fuses along with relays to actuate any auxiliary lighting. The original wiring harness might not be up to the additional voltage demands. Also consider the power draw on the your battery and alternator.
Alan