yes I had around that same amount of fish in my 40g and it said I was overstocked. so I took my then 7 mosquito fish into my 10g. then that dropped it enough to add 2 rescued fish. I would defiantly recommend adding another filter. make sure that you have something to ease the outflow like a fake plant you can stick to the glass in front of it. your neons are going to be your cue fish. for example since I got a 30g the mosquito fish went in there. then I got 5 neons for the 10g to keep a rescued cardinal company. until he passed due to severely damage to his tail fin. when I looked in there yesterday they were a little pale in color and not as bright. so I checked the parameters and the carbon and nitrite was a little high. while in my main tank my tiger barbs were also dull. and that was due to ph. so I did a 50% water change for the 10g with water from my main. and the water from the 10g went into the main. today both the neons and the tiger barbs are back to normal coloration. so if you for now till you get another tank do the water changes and if u see your neons color change, immediately check the water parameters and fix what needs to be done. ask around on this site if anyone has a 40g they are willing to part with or ask your LFS
see if you can do a down payment on one and get it setup. then that way you can pay him or her off little by little. I know my LFS
would do that. but mine is a small business owner not a big store like petco or petsmart so I don't know if you could do that its worth a shot though. as long as you get a constant paycheck you might be able to do that.