That's pretty hard water, I wouldn't do Tetras. Your water is actually extremely hard, even harder than mine and I consider my water quite hard.
Up the number of girls if you can, spreads out the aggresison, maybe 10 or so. You have four Cories in there right now I suggest upping the number to 6 or more, preferably 8 or 10 (but actually Cories are better in softer water, highest recommended for them is 12dH, but you already have four, so. . ) Also Mollies are great for your tank since your water is so hard, maybe increasing the numbers of Mollies too. Around 5 or so, remember to keep a 1:2 ratio, for every one male get two females, this will make sure that the males don't just bully one of the girls to death. Swordtails would also work in the tank, but since you're considering on re-homing them I wouldn't get any more of them.
Yes as everyone has said, seriously try to re-home that common pleco they get so huge, she seriously needs a bigger home. If you're able to re-home her you can increase the numbers of Mollies and Cories, along with more female Bettas.