Do you know what kind of plecs? Common plecs will far outgrow the tank, as will a number of other commonly sold species. 12-18 inches is a conservative estimate. Clown plecs, bristlenose, bulldog plecs, and some others are small enough to be ok, though.
Anyway, first off, since you recently lost a number of fish, make sure to pay extra special attention to ammonia and nitrite readings for a while, as many diseases are precipitated by water quality.
The tank is roughly 26 US gallons. The general (very rough) rule of 1 inch of adult fish per US gallon (for fish under 4-6") makes a good starting place. There's all sorts of factors as to why the 1" per gallon rule isn't great and we could discuss that all day, but if you stick to small fish, it's almost always a bit of an underestimate, making it a good starting place.
I'm going to leave the plecs out of these counts, assuming they're common plecs and will have to be rehomed, I'll touch on them again near the end.
1 blue neon: Is this a blue neon tetra, a blue neon dwarf gourami, or a blue neon rainbowfish? 1.5" for the tetra, 2.5" for the rainbowfish, or 3" for the gourami
4 black widow: 8"
2 zebra danio: 3" (leopard danios are a color variety of the same species)
1 zebra loach: 3"
Current total: 15.5-17, depending on what the neon is.
So you've got a good deal of space, even before you reach the inch per gallon rule. If the tank's been stable for some months, 1.5" per gallon is generally reasonable. With sufficient filtration and maintenance, anything past that comes down to water quality, 2" per gallon is often possible, and some people have successfully exceeded that with some types of fish.
Let's start with what you have:
The blue neon, if it's a rainbowfish or tetra, should be kept in a shoal. At least 6 is recommended. If it's a dwarf gourami, it should be fine on its own.
The black widows are also tetras, and should be kept in a shoal. I recommend 1-2 more.
The danios, again, shoaling. The zebra and leopard are the same species, so 4 more of these would be in order. There are a number of color variants, including blue, gold, and several glow in the dark genetically engineered strains. You can mix and match any of them you want and get a nice varied shoal.
The zebra loach is less social, but would still appreciate 1-2 more of its own kind.
For the sake of simplicity, I'm going to assume the blue fish is a dwarf gourami - that would make a nice centerpiece for the tank as it stands. With full shoals:
Gourami: 3"
6 black widow: 12"
6 danios: 9"
3 zebra loaches: 9"
This will bring you up to a fair stocking of about 1.25" per gallon. With care, you may have some room for expansion. Perhaps one of the small plec species I mentioned, they won't exceed 4" or so, or a pair of dwarf cichlids like rams or apistogramma. If your existing plecs are a small species that would be suitable in the tank, I'd call it fully stocked. If the "blue neon" is a tetra or rainbowfish, you might be pushing it, but with the bulk of your fish being tetras and danios, you're not really packing in the bioload.
