I've looked at the list of fish you say you have in this 12g tank, and every time my head hurts, I go to another thread, then I come back.
I'm fairly new to fishkeeping but I've done an enormous amount of research on the web during the past 5 months on: tank types, tank sizes, filters, heaters, planting, bottom materials, rock, driftwood, water maintenance, how to clean, using gravel vacs, treating different kinds of water, lighting, etc. And that's not including the fish and their needs!!! I'm not fussing at you, just trying to get you to understand that, while fishkeeping is not for rocket scientists, there are BASIC things one must know in order to care for fish in a humane and considerate way.
As already stated, the tank at present is like a can packed with sardines. You say that some will be returned soon to their owners--which ones, and how soon? What will that leave you with? Fish can get extremely stressed out=sick if certain needs are not met. Some fish need more room to swim about in, others do fine with a smaller physical territory.
Perhaps you should wait on thinking about another fish until after the visitors have gone, and spend some time watching your little buddies, figuring out if they seem happy or frenetic. Example: my first tank was a 10g. I got 6 young head/tail light tetras for it, along with several plants. Then I added a mystery snail for algae control. Then a clay pot, then a little resin "rock" bridge for them to daredevil under. A few months later, the tetras have grown at varying rates, none more than 1 3/4 inches (just one), and even though I've set up a 55g for cichlids, I'll be getting a 20 or 29 for the tetras since they are feeling crowded to me. So, I hope this is helpful for you--you just might want to get another tank for another fish, or split the present fish up into two tanks, etc. Best of luck!!! Just my 2 minnows' worth.