ok, so, as far as the tetras go, they would feel more safe and secure that larger the school is. i always keeps schools of 6 or more. do they absolutely need to be in a school? no. plenty of people keep them in pairs and singly and those fish live a nice, happy, healthy life with no problems.
as far as the pleco goes, you can always re-home him and pic up a bristlenose pleco. they only grow to about 4' or so. or some otocinclus cats. they only grow to about 2". of course, you could also add a cory cat or two. all options for down the road, obviously.
as far as your nitrate issue, try rinsing out your media in some old tank water. or some dechlor water. that should get your nitrates down. not that they are really excessive for a planted tank.
now as far as the plants go. i need to ask a couple questions. do you fertilize the tank? how long have you had the airstone in there? what kind of co2 are you adding? and i hate to sound mean, but if those are the same plants that you have had for a year or so, they are just surviving, not thriving or doing ok. lol they should be HUGE. i have had amazon swords grow from 4 leaf plantlets to 2 foot tall 2 foot wide monsters within 2 months.....lol