You can pour sand on top, yes - but be sure to turn off the filter first and let everything settle down before turning things back on again.
Food decomposition does cause ammonia to be in the water, but I don't know how much you would need to put in there to keep a decent colony of bacteria alive. I'm sure if you posted this as it's own thread, someone would be better able to help? Maybe you can put one of your betta or the shrimp in that tank until you're ready for your new friend. I hate to stress out the others by moving them in and out and in again - but it would really just suck to have to cycle over again :)
Whatever you decide, if you're careful, I'm sure it'll be just fine! Good luck!