Thought I would post an update.
Tank is doing really well, down to 9 Cardinals - no trace of the missing fish to this day. We added 30-40 shrimp on Saturday...I think there are 10 left. Cardinals are looking super fat, and several shrimp carcasses have turned up, besides all the shrimp gone missing.
Shrimp regularly get sucked down into the sump...seem okay with it, for the most part, but annoying to have to keep moving them back to the display area. They won't last long at this rate, anyway, kinda sad that the Cardinals are such angry fishies. Not like they don't get fed...they were growing very nicely with my feeding schedule.
The good news about the tank is that it has been a month since the introduction of fish, and I have yet to do a water change, or even consider one (just refilling the sump ~0.5G every other day or so. The ammonia levels are 0, nitrate levels are 0 (strange for a planted tank...they were ~5-10 when I introduced fish I think), and water is crystal clear. Will probably do one once all the shrimp get eaten. Maybe.
Bottom line we enjoy the tank, but very sad the shrimpies won't work out. They were a lot of fun to watch for the first day.