The spectrum and the lumen output should be printed on the bulb somewhere. Standard T-12 tube lights (larger in diameter than T-8 or T-5's) are typically what you would find in a shop and are not suitable for a grow. With T-12's the spectrum of light is pretty poor for growth and will result in leggy plants with lower leaves that often die due to the level of light penetration. Some T-12 balllasts will accept T-8 tubes if they are electronic ballasts. Simply adding T-8's to a older T-12 ballast may result in bulbs that do not light, flicker or even burn out the ballast due to incompatibility(fire hazard). Look at your ballasts carefully before buying new bulbs. Myself, I would invest $50 -$100 in a 2 tube T-5 ballast for a more efficient first time set up supplemented with daylight CFL's, all of which are available in 6500k spectrum.
Plants (like all of nature) are all about competition. Be it for water, light or nutrients plants grouped tightly together will soon begin to stretch for light and one or more will dominate the others as they quickly outgrow the competition. This would truly be unfortunate in your situation as one or more of the plants is likely to be a male and the growth of any potential female may be stunted. If you do have one or more males you want to be able to get them out of there fast, not stressing the other plants trying to extract the root system without disturbing theirs. Coupled with the fact that the longer that you allow them to all stay in one pot, the greater the chance that the roots will become entangled together making removal later on very risky and stressfull to all the plants. I cannot emphasize this enough. Separate your plants now or learn a hard lesson later on.