This was the way to do it before the Tempo Detection feature. Yes, you can tap along into a midi track to make your tempo map, but Tempo Detection does this for you. If you choose the Merge Tempo feature, don’t worry about Detect Tempo. They achieve the same result: a tempo map.
I think what you need to do is once you’re happy with how the detection took place, that is, each 1/1 bar is a beat of your guitar track, change the time signature to 4/4.
I imagine any imported midi track should line up, but you may have to quantize the parts. And since every single beat is a bit different in timing, you’re going to need to quantize the SD parts as you move them around. Because, each time you move the part, the timing will be different.
Say you were to import a midi part into measures 1 through 4. Now, you want to use it for measures 5-8. Well, measure 1 will certainly be different in timing than measure 5. And, measure 2 will be different than measure 6, and so on. You’ll need to quantize the clip each place you lay it on the timeline.
I would suggest though, that once you have your tempo map complete, you use the Set Definition from Tempo (pg 476) to straighten the guitar part to a fixed tempo. Laying in midi parts and tempo sync’d effects will function more consistently this way. Try using a tempo sync’d delay on a variable tempo part…
I believe though, that if you “Set Definition”, once you are happy with the way the SD midi parts align, you can switch back to the tempo track and you’re variable timing should still be available…I think. I haven’t done this myself.
My use of Tempo Detection is bringing in an existing song, create the tempo map, and “Set Definition”, then turn of the tempo track to align the imported audio to a fixed tempo. Works great! Ok, sometimes you need to do a little jiggery-pokery to get the feature to work…