well, since you don't seem to want to wait for someone with good advice, i will tell you my opinion...
i have never had the fish that you described, but i think it is a general no-no to put bettas with tetras, as they tend to be nippy.
depending on how many of each of these tetra that you have, your tank is probably fully stocked without the betta! especially with a common plec. i am guessing that you have already been told he will get too big for a 10 gal, as you mentioned he was a juvie!
hope that helps