Slipstream is a drive, but it also requires modifications to the shielding of ships and to the ships themselves to be a workable drive--the Typhon Pact needed to acquire Dominion shield technology as a precondition for deploying slipstream drive. As for the question of whether or not the Federation (and the Romulans, and the Breen, et cetera) had slipstream research programs before Voyager came with its technology, I'm inclined to think that there were at least research programs in the more advanced Alpha and Beta Quadrants towards advanced propulsion methods which happened to be compatible with Species 118's slipstream.
Heck, you can modify a standard Starfleet deflector dish to generate a slipstream conduit, as we saw in VGR. So it's not a total start-from-scratch technology that would take decades to develop. It's more a new application of potentials that Starfleet vessels already have. And some shapes and sizes of vessel are better equipped for it than others.
Just found this in another thread - it's an early design for Voyager. I must admit that I like this - more than the one used. I'd like to see this appear in Treklit, possibly as part of the 'Voyager' fleet...I presume it would be suitable for Slipstream adaptation.
I must admit, I've always loved that proto-Voyager! Pics of the study model can be seen here. (I think they used to be at starshipdatalink.net too, but that site appears to be down... for now? or permanently?)
Starfleet would've needed to have researched the technology further from that point, since as you recall Voyager just made it into slipstream space, but when it did, the stresses were so high that Voyager's integrity field was just holding the ship together.