Betta fish

The Betta has been pretty well covered now but I was thinking - just in case you might have been thinking of any other Anabantid fish that unless you were to defeat the object of the aesthetic appeal of the tank - as suggested - by keeping the level a little below the top any of these would be out too. This would include other Bettas, Paradise Fish,Gouramis and the like.
But I wanted to add that any surface-breathing Catfish (all the Corydoras for starters) wouldn't be able to be kept either. Unless these can get to the surface they will die too.
I learned this lesson the hard way many years ago.
As to the suitable plants, any of the 'low light' plants should do pretty well - the Anubias already suggested should thrive and the small tank size will have to be a consideration. You might try Cryptocorines, mostly very hardy, they will thrive in lower light levels too and once established are hardy too. The only drawback is that in the main, under those conditions they can be slow-growing (which, in a smaller tank might just be a bit of a blessing).

Alternative options for fish? You might want to consider a shoal of a smaller species of fish, I like the (what used to be called) Microrasbora Galaxy, I think these are now called Celestial Pearl Danios. I think another good idea might be to have a small shoal of Tiger Barbs (the original striped variety, not the manufactured awful abominations so often seen nowadays). I would add that my preference would be for this to be a 'species' tank so they wouldn't get the opportunity to harass any other species...or you might consider other small Barb species instead.
But, this is your tank and there's me saying what I would keep in a small tank, ignore me - I'll go away!!!

John

Gerry,
I would suspect your 'Edge' will be a bit limiting to the amount of Barbs you could stock it with - which is why I was suggesting a 'species' tank.
Tiger Barbs, in their different guises will grow to over 2-1/2" and Rosy Barbs are claimed to have a maximum size of 6"!!! - I, for one, never saw a 6" Rosy Barb so perhaps the countless generations of far-east bred Barbs has seen them stunted! - I once had a pair where the female was well over 3", but that was SO long ago it seems like a different lifetime.

All this is adding up to my opinion that a mixed Barb community tank might best be saved for a time when you can get another tank. Unless...you might consider some of the smaller Barbs such as Cherry and Chequer Barbs, the Striped Barb (Lineatus) could be another contender as they are pretty well-behaved too.

Many of the Barbs come from fast-flowing highly oxygenated water so their oxygen requirement would be greater too - you could possibly count the Tigers (in their different 'bred' forms) but most certainly the Rosy Barbs would and so the general stocking guidelines would have to be 'stretched'.

Gravel or Sand? I see you have decided on Sand, a very popular choice nowadays. I think, although I wouldn't be the greatest authority on Plant growing (pretty good at killing them off though), that if you were to get some of those plant tabs (I used to like the 'Hobby' ones, but haven't seen them over here anywhere) and put at least one under the root of each plant, that should be enough to get some nutrition to the plants until the sand gets to the stage where it can fully sustain the plant growth.
But perhaps someone better versed with plant growing knowledge can give better advice here.

Good luck with the tank, whatever choice you end up making.

John