I don't know how long the eggs will last but it will be a long time. Just use too much in one go. You will get used to hatching the right amount eventually. I use about a third of a plastic teaspoon but that will be too much for you. Try a quarter and see how you get on. I use the leftover brine shrimp on my other tanks.
Someone might correct me on this one but I thought that Selco was similar to HUFA. I have the HUFA and although a very good enrichment it is extremely messy and stinks. It pollutes the tank quickly even though I used to rinse it well. I still have some but only use it occasionally.
Here is my brine shrimp set up with the enrichment vessel next to it:

Joanna x