I know this Thread is OLD, but, I just finished a successful fish-in cycling using TSS (Tetra SafeStart)! We conditioned our Marineland Eclipse Hex 5 w/Prime, waited one day and then added a 3.38oz bottle of TSS and our new male betta (Stormy). AND, as the Tetra Rep said in their "answers", we didn't do any W/C's for two weeks. During that two-week period, we did water parameter tests and everything was fine. Day after that "two-week period", we done a 25% W/C w/Prime added. Our water parameters are really great and so is our betta, Stormy.
Tetra (SafeStart) and API (QuickStart) make this stuff exactly for cycling a tank with fish inside. There was absolutely no reason for us to do the "fishless" cycling when these two products are sold for the purpose of fish-in cycling. However, I've read that the bacteria in QuickStart isn't nearly as good as in SafeStart..........just what I read.
Do it right and a fish-in w/SafeStart will work perfectly.