I will post quickly so you have a response and come back and edit it for more detail........I am otherwise occupied at the moment lol
Okay, light is what drives plant growth and the need for plant nutrients especially c02.
Since you do not have c02 now, I would suggest using only 1 or 2 (max) bulbs in the new fixture.
The T5 will be newer and brighter than the T8 on a one to one comparison, so even just replacing the 2 current bulbs with T5 will be a big increase in the light THAT THE PLANTS ARE NOW USED TO.
This is an important point.
When you increase the light, the plants will want to grow faster, and so the need for c02 and macro/micro fertilizers like nitrate and phosphorous etc increases.
The plants need TIME to adjust to this, and while this is happening algae which is always in a tank, uses this time to take hold and spread.
So, using 6 lights IMO would be way overkill as you suspect. While you could do it, it does not meet your goals, and would require a lot of c02 and nutrients to keep up with the light.
So, with your new fixture if you could suspend or mount it in a way that would enable your to raise or lower it at your desire, this would help you adjust the light as you want, and could use more bulbs.
Also the spread over the tank is important so keep this in mind when you remove a bulb or two........You want to eliminate any dark spots unless you want them there.
Anybody know how to post a thread
If you look at each forum, at the top left somewhere there is a New Thread button. Just click on this and you can start a new one with your subject, etc.
As you have time, this thread may help to understand the importance of c02 and how to use fertilizers should you go that way. It also includes a section on non-c02 low light as you want.
I hope this helps.