Oh dear. Very sorry for your loss of the fighter and Tetra. They must have been beautiful to marvel at...
Now, what you have there is quite a sticky situation. I personally advise against putting the loach in with the pleco tank as they would both become extremely territorial against each other, assuming it was a common pleco. Plecos are recommended to be the only bottom dwellers and algae cleaners in a tank, and putting this loach, whichever species it may be, would cause stress and even potentially be fatal to one or the other in a tank without alot of cover and hiding caves... So my advice and honest opinion would be to not combine the Pleco and Loach if you can.
Now I am assuming that you have 3 different tanks in all, the betta and neons, the quarantine and the pleco tank yes?.
Betta and angel = death and doom.
So the best thing to do in this case, If i have my facts right in the tanks area, is that you should put the betta with the pleco and catfish. THe angel fish can go with the neons and you could give the loach a try in there.
Neons are fast and the angel is aggressive, which is quite risky but better than letting your betta be chased, your betta doesnt seem to have a typical overly-aggressive fighter characteristic, rather, more peaceful. So give it a try and monitor carefully. I am currently keeping my betta with 5 neons and a bn pleco as well!
-Aquaone 620 90L Main tank (Undergoing Nitrogen cycle)
-MarineMaster 8L spare tank (Nigel-Betta Splendens Crowntail) ~ Deceased 2015