Depends what we interpret as the fish being "happy"... Sorry I think I'm going to get a bit philosophical here...! I would personally not keep any fish 'alone' unless it's nature is to be on its own and it is aggressive with its own kind for example. But...
Obviously we can't ask a fish if it's happy, so we interpret behaviour, coloration etc as whether it's happy. Certainly there doesnt seem to be much impact on factors such as these from keeping sparklers singly. But you could probably argue this for neon tetra, they are a very loosely schoaling fish and if kept singly it's behaviour won't change significantly, neither will it's coloration...
But back to sparklers... they are a gourami which appear to be inquisitive little fish and seem to interact with the tank contents etc which we interpret as showing intelligence. Does this mean they benefit from company and would get bored without it, or that they dont need company? Other gourami traits include that they are pair forming and don't live in huge shoals naturally. So this could indicate they are happy being on their own... or that they are best in pairs...
The simple answer is we won't truly know if we are providing the right habitat for our fish and simply experiment to see how they fair in the cages in which we keep them but we do recognise our responsibility to provide a suitable habitat for our pets which is great... oh dear I'm going off topic again...
In thie case of sparkiling gourami the general consensus seems to be that they are ok kept on their own and they show no significqnt physical or behavioural differences when kept this way.