Do it. You have to start somewhere, and if you are ready for this, then do it rather than wait to do a bigger batch which might not happen for a while. I'd suggest you add some fresh sacc yeast with the dregs, and get the freshest dregs you can. It is likely the sacc yeast will be dead in the dregs due to age, but the Brett, Pedio, and Lacto will last very much longer. They too will eventually die, but you are looking at a few years before that happens.
Make the two single gallon batches and see where it goes. When you are ready to make a larger batch, all you need to do is rack the beer off one of these cakes, and add the cake to the larger batch. Keep these single gallon batches on the yeast cake (no secondary) until you have a need to use the cake.