Shrimps graze 24/7 and if you have a small tank without much surface area for biofilm to grow fast enough, it is risky. In addition if you have a lot of shrimps they can strip the biofilm in your tank in a day or 2.
 
When shrimps start to go without sufficient biofilm it triggers cannibalisms, which means they will start preying on each other. This is detrimental if you are doing a selection process as your stock shrimp will be at risk. Hence, having a good source of grazing material not only is critical for a small tank, it enhances the probability of breeding as well.