About the galaxies, from my experience they will appreciate some hiding spots and the males do get aggressive toward each other. Some stems in the corners will do them a lot of good.
+1 to what Mumford said. They really do appreciate hiding spots.
And ya about the co2 from what I've researched single stage is OK but you're going to want to keep an eye on the tank pressure and as soon as it begins to drop you're going to want to refill the tank before the liquid co2 turns to gas and causes eotd. There's a lot of debate on this but I feel you should be safe vs sorry.
That being said there are a number of people who use paintball co2 setups and single stage regulators without issue. For about 150$ you could purchase a number of setups but I commonly see Milwaukee single stage regulators and aqua tech. They both run around 90-100$. Do a search on the sales forum and you can find people selling paintball setups for around 100$ (including the tank you will need to purchase elsewhere). Just keep in mind that you will need to keep a closer eye on a single stage setup and I would recommend setting up and dialing in your system before adding any livestock.
Last edited by theericafish; 04-22-2013 at 10:38 PM.