I used to use the 4 foot/four bulb fixtures but they created a LOT of heat. I switched to a 400 watt HID with a switchable ballast that has a 10% boost option so i run 440 watts. I was using, I think 50-54 watt bulbs in my T5s. i now use 1 of those for seedlings, cloning and beginning vegging for two weeks then put them under HID counting that as the first week.
I assume that your fixture is what, like 15,000 lumens? Mine with 4 bulbs is at 22,000 lumens. your set up sets about a 2 square foot area going by 4' x .5'. Mine is 4 sq. ft. being 4 x 1. So you are looking at about 7500 lumes per sq. ft., rough estimate. This is good for vegging. use blue for veg and red for flowering to simulate seasons.
you are also running about 45-50 watts per sq. ft. which is great for beginning stages of growth including vegging. Adding two more 4' T5s would still be about 7500 and still about 55 watts per which would get more light to the under growth and produce vigorous growth. i had a GREENHOUSE co. White Rhino that grew like a head of kale under t5s. granted I had a 4 bulb fixture over the plants and one mounted to the side of the wall to bath the sides in light. This burned the plants a bit though.
I would advise growing more like 4 plants in one or two gallon pots and keep them short. Like change lights whe plants are 10 inches tall. Top them leaving four tops as this will really bush them and keep them shorter w no real loss in yielad. if you do grow 6 plants I would advise either another 4' fixture w/ 1 or two bulbs to hang right next to the 2 bulb to give a larger area of light. or you can get a 250 watt CFL with an Econowing reflector that has a built mogul socket for like $80, hang it at the end of your fixture and you will be able to get 6 plants easy.
I really advise upgrading to HID lamps if you can.