Six fish would be pushing it...the tank might then go through another small cycle and there might be some ammonia. I would either start with the three guppies or the three cories
. Test the water daily and if everything is fine you can add another group of fish...maybe in about five or six days?
You can have all males as long as you have a bunch of hiding spots
. When I had my guppy tank I only had males. Yeah, cories like to be in groups. Keep in mind that if you get different kinds of cory cats they wouldn't always shoal.
I personally feel that different kinds of fish do indeed need different kinds of food. Some people just use tropical fish flakes (you shouldn't use goldfish ones...they are different) for all the fish but I don't agree with that. After all, you wouldn't use dog food for cats, rabbits, and guinea pigs, right? Research the kinds of fish you want to find the best diet. Guppies need a diet that is really high in protein. So besides regular fish flakes/pellets it's a good idea to supplement their diet with betta fish pellets or some other high-protein food. Frozen bloodworms and brine shrimp are also great sources of protein...plus they are much healthier then flakes. Frozen krill is another healthy meal for guppies that will bring out their color. Frozen mysis shrimp is another food I like to feed...it's really healthy and tasty (for fish, lol). Guppies aren't as likely to nibble at plants if you give them vegetables sometimes...mine liked carrots, romaine lettuce, and cucumbers. Cory cats do well on a diet of sinking algae wafers, the occasional vegetable (cucumbers and zucinni are good), and frozen bloodworms. Of course they also get bits of what everyone else is eating, lol.
As you might have guessed, I go pretty crazy when it comes to my pets' diets
. I mostly feed frozen foods...you can get all sorts of kinds at the petstore though you may have to ask since they would be in the freezer. Variety is key. I would just pick up a bunch of different frozen foods plus some sinking wafers for the cories. Maybe get some flakes for days that you don't have much time, lol. I use flakes and pellets if I'm in a rush and don't have time to defrost stuff. They are also good if I go away...very easy for family members to measure and feed (though the saltwater fish absolutely need frozen foods...I'm picky with them, lol). But seriously, frozen foods are sooo much better then flakes. You'll really see a difference in energy, color, and overall lifespan.