I wouldn't pick elodea as a species to introduce to your pond. There are far better options.

If you have only BG, they are not pregnant. They may be egg carrying or milt carrying, but they do not bear live young. Stocking 1 LMB now would be fine. If you stock 2 or more now, you run the risk of getting males and females and ending up with a reproducing population. You decide if you want that...