People use the word "co-factor" in a variety of ways, some meaning "you can only get AIDS if you also have this factor present", others meaning "X will speed up progression to AIDS, but it is not necessary for the development of AIDS". Other than HIV, nothing has been found to be a necessary co-factor (and people have looked!). Plenty of things fall into the latter category - all sorts of differences between people with HIV (genetics, age, nutritional status, other infections, viral strain.......) are known to influence the speed of progression.
"Although HIV is the necessary and sufficient causative agent of AIDS, genetic and environmental factors markedly influence the pace of disease progression."
Nobody is suggesting that HHV-6 is necessary for the development of AIDS. This is difficult to study in humans, as nearly all humans have been exposed to HHV-6, but Gallo's lab did find an animal model to study it in:
The results? Both groups of macaques progressed to the SIV analogue of AIDS - the group with both SIV and HHV-6 merely progressed faster. This does not make HHV-6 a necessary co-factor.
"No statistical difference could be found between HHV-6 prevalences or HHV-6 geometric mean titers obtained for each geographical area in the different HIV-1- and/or HIV-2-positive and HIV-negative groups. HIV patients with clinical manifestations of AIDS did not differ in percent seropositivity or distribution of titers from the HIV-asymptomatic patients or HIV-seronegative patients."
Even the 1995 interview with Gallo made the same points: it is worth reading the entire interview rather than cherry-picking quotes.
I'm not sure what sort of "research" you've done on HIV, but since you have access to a lab, it should be very easy to convince yourself that HIV is indeed cytopathic.
Ch. 13 would also interest you, but you'll have to buy the book to see that!
In general, you'll want to read a lot more about immune activation and bystander apoptosis before you decide that your understanding of HIV is complete enough that you're qualified to reach a conclusion about whether or not HIV alone is capable of causing the profound CD4 T-lymphocytopenia known as AIDS.