That's why I have discouraged trying to bring up common guppies to show size. While, with out-crosses, it wouldn't take the decades that it first took to develop a delta tail guppy, we may be talking a number of years!

And if you were to approach your fish dealer and ask if they ever see albino balloon belly mollies at the wholesaler or on the inventory lists, they probably would say yes. It might take them a time, but probably you could have them in less than 2 years that way. ;)

If the Punnett square used below accurately reflects what may happen in the second generation, the time may be less. Please realize that I'm in really unfamiliar territory here. If anyone happens upon this conversation and is familiar with genetics, please feel free to jump in and correct or confirm what is suggested below.

I was really frustrated trying to Google something relevant upon the genetics of Balloon Bellies and Black Mollies. Somebody has crossed them before I'm sure, but even with a little use of the advanced search, didn't find much. Of course it was also too late at night for a non-geneticist to be wallowing  through genetics.

You are dealing with at least two genes, one for the truncated spine of the balloon belly as opposed to the regular spine of the black mollie. There is also the color factor, going from the black sport with a heavy concentration of melanin (the black pigment, possibly in melanophores) to a sport without any melanin or other color.

One article noted that when two characteristics are controlled that both may be carried over to a cross when both genes are located fairly close on the same chromosome (which will carry many genes). However I seem to recall that black mollies of 46 diploid chromosomes. That means they are in 23 pairs. Entirely as an amateur, I'm guessing that what controls color and the shape of the spine are not closely located upon a single chromosome. More than likely they are even carried upon two different chromosomes.

Some things (like male colors in guppies) are on sex-linked chromosomes. That is a little easier to negotiate because there are one or two chromosomes usually to deal with. But the bubble shape and the albino and black colors we are dealing with factors which are not sex linked and might be associated with any of the chromosomes.

Oh! And I'm making an assumption that the bubble spine and albinism is each due to one genetic factor. Sometimes more than one gene is involved in a fish's characteristic. (Sigh.) I'm guessing that the the spine is one factor. Albinism may be one or a couple of factors.

In a basic one gene difference, a cross will produce a first generation which is all the same. If one of the genes is dominant and one recessive, all of the off spring will show the dominant characteristic. For instance if you crossed gray bodied guppies (GG) with gold bodied (gg) guppies, the first generation (Gg) would be all gray. If they were crossed the next generation would be 1/4 GG and show gray. It would also have 1/4 Gg and 1/4 gG (actually still Gg) and would also be gray. 1/4 would be gold (gg) if they, as the sport, were hearty enough to survive. (Some like albinos are sometimes a little weaker and so 25% of the fry don't all make it.)

Now sometimes (human "breeders") will risk inbreeding problems to get their new strain more quickly. If the original father was gold (gg) they might breed him (back cross him) with one of the daughters of the first (F1) generation. If a gg male was bred to a female Gg theoretically there would be 50% Gg fry (gray) and 50% gg or gold.

That is also a good way to check to see if the characteristic you are breeding for is controlled by one gene. If more than one gene was involved, there probably would not be a 50-50% ratio in such a back cross.

Someone with more mathematical prowess will want to check it the following is correct. A Punnett square can be used to guestimate what you will get in a second generation cross. The reference below suggests that if you did a second generation cross you might get 1/16th showing the pattern you wish to achieve. Look at their estimates for a dihybrid cross. They also give instances where that may not be true, You will need to save quite a few fry and then probably pass most of them on to your pet shop gratis. Yeah and they may be used as feeders. :(

http://en.wikipedia.org/wiki/Punnett_square#Typical_dihybrid_cross

If you get albino/balloon bellied fry, sort them out into their own aquarium space as soon as you can. With luck you may have enough to start a third generation, which "should" breed true.

I wish my experience or knowledge here was more reliable. Hopefully this helps a little. Please keep us posted.