Your tank is 15 gallons (right?) so forget the krib. It wouldn't get along with the guppies anyway.
Also, you are fully stocked. At most you could consider getting a few more rasboras (2 or 3) as they are a schooling fish so like to be kept in groups of 5-6. BUT, wait until the tank is cycled (that's when you have 0 ammonia, 0 nitrItes, nitrAtes below 40 - buy a test kit and do weekly water changes to keep nitrAtes down afterwards).
The siamese flying fox needs a bigger tank as it'll get to 6" and, in this small tank, on its own, it will get aggressive and may kill some of its tankmates. If it is truly a SIAMESE flying fox, it needs to be kept in a group and a lone one can become very aggressive. If, on the other hand, it's a flying fox, it needs to be kept alone but still needs a much bigger tank and will also get to 6". Could you post a clear picture (taken from the side) of this particular fish so I (or someone else) could identify it?
If you could return this fish to your LFS, you'd have more space for a few more fish besides the extra rasboras BUT nothing bigger than 3" at full size and nothing even remotely aggressive (kribs being aggressive enough so that you cannot keep them in there).