Oceanmist...
Angel fish can reach 10" from top to bottom fin. Recommendations are generally one pair per 20 gallons of water volume...and preferrably a tall tank.
BTW...I have successfully kept angels and neons together. I have kept many 'would be predators' with 'would be prey' successfully with many species. I have found a trick
if you ever get the itch to try it out. Best in a larger tank than 15 gallons though.
There are lots of interesting and different little fish that would reside nicely with the neons in a 15 gallon. Personally I would not pick any of the larger dwarf gourami's in that size tank...like the powder blues and cinnimons. The little honey dwarfs would be sweet. Sparkling gourami's only get a single inch...maybe 1 1/2???. Even a betta would do ok...but watch for the neons. Better off with a female betta or a short finned male. As passive as neons are...they too may see the long tail of male bettas as a tasty morsal.
Cory catfish are perfect little bottom feeders for small community tanks. Otocinclus are great little algae eaters...and safe with live plants too. They will eat the algae off the plant without destroying the plant.
If you want to get a little unique...hatchet fish are really interesting and do best in a natural tank. Silvers get just a little bigger than the marble hatchet and a bit hardier too. These fish are trippy.
Another unique but helpful critter is the
FW clam. These things eat nitrate. Though life spans are rather short (3 to 6 months average), they are inexpenisve enough to replenish the population. Cost is generally $2 bucks per. Caution...do not house with plecos. I have pictures of plecos macking on these clams. Surprised me!!
Pencil fish are neat...rasboras...you can add little shrimp. There are small species of loaches to help with any snail problems from the live plants without terrorizing the tank. Dario botia is one of them. Affordable too. One of the coolest shrimp I know of for a little tank are the flower shrimp. They get about 4" and instead of claws...they have these flowerette looking hands they use to catch food debris in the moving water.
Who says variety is only in Saltwater???
Khuli loaches are good for keeping the substrate sort of 'ventilated' and sifted. Keeping substrates like flourite from becoming caked. Spaghetti eels are even better.
I would stay clear of Bolivan Rams...mostly because of their size. They are a larger species of ram. Blue rams, gold rams and the apistogramma species of dwarf cichlids are excellent. Best kept in pairs.
To best utilize the most area of the tank without causing problems with territorial aggression is to get fish that occupy different areas of the tank. Top feeders, middle feeders, and bottom feeders. You could even go over the normal recommended populations so long as you have good bio. Fish that live in other areas tend to clash less. Neons are middle feeders and therefore occupy the mid section of a tank. Most catfish stay at the bottom...and top feeders like hatchets stay close to the surface. Loaches dig and live in the substrate.
Just like with a natural marine system...you can achieve the same type of virtual self maintained system by keeping fish that help you to gain maximum benefit. The reason for marine being very difficult is that most marine animals are not adapted to take on sudden changes. Freshwater however go through drastic changes all the time from season to season. They can tolerate a bit more 'abuse' than their marine counterparts. The oceans are vast and experience very little change.
HTH.
