I second everybodies opinion about taking the babes back and doing a fishless cycle, it's a he'll of a lot easier.
If you persist with a fish-in cycle you probably find you'll need to do 2/3x water changes a day at 50% each time for the next 6 weeks so it's a lot of work.
Regarding the neons, they need a mature tank older than 6 months as they are very sensitive the fluctuating water conditions which are present in new tanks.
I'm presuming the colourful big tailed fish you mean are guppies, if you get guppies, only get males otherwise your tank would be overrun with babies, they breed every 6-8 weeks or so
Andy