Looking for a Good Centrepiece Fish with Tetras

So, I posted in another thread about my severely neglected tank. (Wasn't able to maintain due to extended illness.) I'm trying to get it back under control now. Unfortunately most of my fish did not make it through the period of neglect. Frankly, I'm rather shocked that any of them survived! Anyway, the first order of business is obviously to -- gradually -- get the water parameters and algae to a reasonable level. I've been working on that for the past few days and things are already looking a lot better. I'm definitely not ready to add any new fish quite yet, but I can't help thinking ahead to what I'd like to restock with once this tank is finally up and running properly again. I have some ideas, but I could use some help. Suggestions would be appreciated!

Here's what I've got:
Tank Dimensions: 48"x21"x20"
Substrate: Pool filter sand
Lighting: 1x 48" 65K daylight T5HO + 1x 48" roseate T5HO w/ 8-hour photoperiod
Heating: 2x 100W heaters set to 24C
Filter: 1x Fluval U4
pH: Was around 7.5 the last time I tested but haven't checked recently due to said illness and lending test kit to a friend.
GH & KH: Out of the tap Ca is pretty much nil since it has gone through a softener, buffering/carbonate however is still quite high (can't remember exact number... in and around 230ish, I think). I suspect the numbers for the tank look quite different now as I've had a lot of snails die off and leave their shells behind. Will update when I get my test kit back.
Ammonia/Nitrites/Nitrates: No idea what the numbers look like at the moment, but won't be adding more fish 'til I can test and am seeing 0/0/<20 anyway.
Flora: Currently anubias and Java fern
Fauna: Currently 4x Black Neon Tetras, 3x Bronze Corydoras

Future Additions:
1 additional filter (Any suggestions? Preferably something that doesn't churn up the water too much as the U4 already seems to create quite a bit of current. Also, I've not had good luck with air pumps+sponge filters in the past, so I'd rather stay away from those.)
Hornwort
Vallisneria
Sword plants
4 more Bronze Corydoras
8 more Black Neons
12 White Skirt Tetras
1 Bristlenose Pleco
~12 Amano shrimp (maybe)
1-2 large-ish, vibrant centrepiece fish

Obviously I want something that's going to be compatible with my other fish, both in personality/peacefulness and in water parameters/temperature. In addition to that, I'd like a fish that's not going to destroy my plants. I certainly don't mind the odd bit of grazing, but I'd rather not have my plants decimated! Something brightly-coloured, moderately-sized (let's say 4-6"), relatively hardy, and non-schooling would be ideal. I'd be open to keeping an individual or a pair, but if I'm going to get more schooling fish, I feel like I'd rather just increase the number of tetras instead.
Any recommendations for a nice, eye-catching addition once my tank is ship-shaped again?

Sorry there's some gaps in my info. Hope you've still got enough to go on to toss out a few suggestions. Thanks in advance!

I'll look into everything mentioned above and see what grabs me (and what's available in my area). Thanks so much for the suggestions! Really excited to finally be getting my tank back on track again.

Hmm... ran into a few discrepancies when trying to research some of the suggestions.

I think rainbow cichlids might be nice, but I'm confused as to how many I should be looking to have. Initial information seemed to indicate that they would be okay kept singly or as a pair, but when I add it through AqAdvisor I get a warning that they should be kept as a group of at least 8 and other sources recommend harems of one male to "several" females. I might be convinced to take on a fish that requires a small group/harem (3-4 fish), but 8 seems like a lot to me. Also, if I add the full 8 through AqAdvisor (just to see where that puts me stocking-wise), I get a new warning pop up saying that they'll likely become too aggressive to co-exist with the black neons if/when they start spawning.
Anyone have any thoughts on this? How many should I be looking to get if I go with rainbow cichlids and are they likely to beat up my tetras?

Rams are also quite intriguing. I'd consider them as well, but have seen a few cautions at this point that Rams may have become victims of their own popularity in much the same way that neon tetras and dwarf gouramis have in that over-breeding/poor breeding practices seems to have weakened them considerably. I've also seen recommendations to start with a small harem and allow a pair to form naturally.
What have your experiences been? Is it generally difficult to find good stock these days? Would I be asking for trouble if I just got a lone individual or a pair? If I did go the harem route, could I leave the extra females in the tank after the male had paired off? Or would I have to re-home them?

I have to say, gouramis also appeal to me, particularly pearls or three-spots. They seem like they'd fit my tank nicely and the information I've found on them has been pretty consistent.
Would I be better off keeping a single individual or a trio? And if I did go with a trio, would I have to worry about aggression if they spawned? (I won't be actively encouraging spawning, but I'd still like to know what I should expect if they do.)

Thanks again for all your help. I really appreciate the suggestions!

@Steven 1: I could be wrong as I've never kept cichlids before, but I think you'd want at least a 30g for them.

Thanks for the additional info. I'll take a look around and see if I can find anywhere that I'd be able to get Bolivian Rams in my area. If I can, then I'll pick something else as my second variety of tetras. Would Bleeding Heart Tetras be appropriate? The info I'm finding is saying that the males can get territorial, but also that they can be kept with non-predatory cichlids. If not, I'll probably go with the Black Phantom Tetras as suggested by Byron.

If I can't find anywhere to get the Bolivian Rams locally, then I may just opt to have larger schools of tetras with no centrepiece fish. (I think I'd rather have the White Skirt Tetras than the gourami.)

Thanks again for all the help and advice!

Oh! And happy discovery: turns out I have not three, but (at least) five cory cats left in my tank! They were just taking turns coming out of their hiding spots and I didn't get a chance to see them all 'til last night.