I think that would kind of be too much for just one vessel. I think they should just add it the way they added the Kumari. As in "Hey they've sided with us a long time ago but suddenly they share their ships with us!"
The xindi indeed joined the Federation, they were on Enterprise J in ~2500. Nevertheless, they could have joined anytime between ENT and VOY or even shortly before STO.
Definitely Voyager! The Delta Quadrant would be awesome to explore, seeing how the Kazon evolved etc. And I'd like to see admiral Janeway! Hell, I DEMAND admiral Janeway!