Another question I have is - lets say I want to use an acapella at 95bpm, is there any way to use that acapella in a 126bpm house track? Downloading the audio file will keep it set at 95bpm right? Must I alter my own track's bpm so that it fits the acapella? And if so is that the case every time?
Yes I have stretched vocals many times using
Ableton for mashups and for producing. I put Warp Mode to Complex Pro with Formants at 100%. This setting ensures a cleaner crisper sound for your vocals. I then use the metronome or a kick drum and drag warp markers so it fits. I find the auto warp isn't as accurate as me doing it manually. It doesn't take very long once you get good at it either

.
Side tip: If you want to do pitched down vocal (skrillex style...) settings are: Complex Pro, set your transpose to -12 (one octave down) and change the formants to ≈0%. Once you change the Formants you will notice how clear the pitched down vocals become. For
pitched-up go to +6 or +12 depending on how chipmunk you want to go.
I would pay attention to some of the remix contests from Beatport, Indaba & WAVO to obtain vocals and track stems for remixing. This website
http://www.remixcomps.com/ does a pretty solid job of putting together most of the remix contests going on presetly.