Hi Jeff,
It really depends on the brand of PC bulbs. For instances, it takes a workhorse 7 ballast to drive 2 55W panasonic PC's while 2 55W Sylvania PC's can be drive from a workhorse 5. For the past 3 months, I've been using a workhorse 7 to drive a 24" 20W 6700K t12 as well as 24" 20W t8 marine glo lamps at 110W each. They are bright!!!
You can use these ballasts to drive different lamps in different config. Check out
Fulham's website
Disclaimer: The above mentioned lamps are overdriven. I am not liable for anything that happens if you attempt the same