Yeah, if the thickening delay has only one input and you want to use the TF in stereo, you'll need to put it before the TF. Just be aware that the repeats the TF adds, when the TF is active, may lessen the thickening effect, depending on the settings and what TF effect you're using.

According to the Haas effect (theory of sound localisation), more than 50ms and it starts sounding like an echo. Personally, I find anything more than 30ms to be too much, but your ears may tell you something different. I think 20ms should work for what you want, but you'll have more control and more choices with something that can go shorter. There are some really colorful sounds between 1ms-15ms or so.

I definitely understand your desire to use an analog delay here, and you may like what it does to your sound. Since one side's completely wet and the other is completely dry, the difference in the two will be much more noticable using an ADL instead of a DDL. An ADL's dry side may be noticably brighter than the wet side, and because changes in tone can be mistaken for changes in volume, this may shift the stereo balance when turning the thickening delay on or off. A DDL will be able to reproduce the wet side in a more identical fashion.

I think my best advice would be to try both a DDL and an ADL and go with the one that sounds the best to you in this application. The most important thing is that it sounds good to you.