Certainly there's more to making different rhythms fit together than just matching tempo (matching time signature for one), but if you recorded (guitar?) to quantized MIDI drums, and your performance is reasonably tight, any MIDI drum loop you being in later shouldn't need to have its tempo changed. If it doesn't sound right, it might just have the wrong rhythmic emphasis or maybe it was meant for a different time signature. Sometimes this actually produces something interesting, but sometimes not so much.
One possibility, since you're composing in the PRV, is that you're creating your initial throw-away MIDI clip using the wrong snap resolution such that what you're thinking of as a measure isn't matching a measure in the timeline. Or maybe you're converting it to a groove clip, and the Beats in Clip value is wrong so it gets stretched when you convert it, slowing it down. Before you start recording audio, you should make sure that your initial MIDI clip keeps good time with SONAR's audio metronome when enabled on playback.
Two approaches that could help avoid all of this are using the Step Sequencer to create that throw-away drum loop or recording an extended pattern using your e-drums played to a click, and then quantizing them so you have a really tight timing reference for recording other parts while retaining the more natural dynamics of a real-time performance.
If you can play e-drums, I recommend you avoid getting too wrapped up in trying to humanize sequenced drums, and focus on getting your e-drum performance tight enough that you can just fix the "clams", do a little percentage quantizing, and call it good.
If you could share a download of an example project that's not working as expected, it would probably be a lot easier to figure out where you're going wrong.
Hope that helps.