Ideally you wanna keep the nitrates from exceeding 20. What kind of filter is on there? A stock kit filter might not be able to keep up with the bio-load of 8 livebearers. Certainly adding them all at once to a tank that only has 4 neons will likely cause a mini cycle. I think you can do it, if you really need to, it will be a bit overstocked, but if you can add them gradually that would be ideal. If not, you'll need to be testing the water regularly and keeping up with pwc's and dosing with Prime as needed. Maybe you could pick up another little filter to run on the tank as well and stuff it with media from your moms 60 gal. That way you could avoid the mini cylcle and definitely have enough filtration for the bio-load of those specific fish...