I am now ready to add fish into a 15 gallon tank. I was at my local pet shop and my daughter really loves platys. I know I need to add only a few at a time. Is it ok to mix a few types and maybe a couple of fancy guppies ? (i figure. I should be able to fit 6-8 in this tank size) Also what is best to use gravel or sand? And lastly in a few weeks can I add a bottom feeder and a few shrimp?