Tank to Fish

Question
Over the past year my toad and newt tank with just 4 inches of water as evolved into a tank with 7 1/2 inches of water. The Tank's normal size is 30 gallons of water and the complete height of it is 16 inches. There is still some land for my toad and newt in the top right hand part which it roughly 1/4 of the area but there are caves i made under the land so the fish have almost all the area. Now the fish i keep in my tank are five: 2 dwarf corydoras about 1 - 1 1/2 inches each, 2 dwarf puffer fish which combined equal about 1 - 1 1/2 inches, and a 2 inch goby. Then i decided to get a green cobra guppy and it hit me... my other fish arent that colorful besides a black spot to go with there grey... So now i want to get a school of three guppies so i went to get a powder blue guppy and unfortionately for him my cobra guppy didnt like him to much... needless to say he died. Now then the reason you had to endure my stupid story is to give you a better idea of where im at. Now heres my question. To my understanding guppies were a schooling fish, but then why would mine kill the new one. It came to my attention of course after the fact i got the new fish that you must get all your fish of the same species at ounce or they would kill eachother. I want to ask you if this is true, and if i traded in my cobra guppy for a new one and started over with a new trio of guppies if they would kill eachother. If this is indeed true that the fish would kill eachother if not put in all at ounce then why was it when i got my 2nd corydoras that my other one not only did not try to kill him but became extremely happy and is almost always swimming with   him. Anyway thats the first question. Now ive been harrassed so much by my uncle that i have to many fish before i even got the guppy. He says the ratio of inch of fish to gallons of water is 1 inch to every gallon. However, I heard else where it was 1 inch to every 1/2 gallon. Please tell me if I have to many fish and if i could get my dreamed trio of fancy guppy ( and how many other fish i could get for that matter). By the way if you need to know my newt is about 2 - 2 1/2 inches and the toad is about an inch but he is a fat toad. Anyway i checked my PH and it is 7 I also have quite a bit of plants so they help it alot plus i plan to get more plants. I also wanted to get a decant sized shrimp but im not sure if my goby will pick at it or if it would try to eat the fish or just pick at there fins. Well sorry for being long winded, but i figured im better off asking you all of this now instead of sending 30 consecutive emails. I hope your 40 years of experience can help me.
PS: if you know a nice web site or magazine or something like that where i can look at all differant species of fish with nice colors id appreciate it if you told me. Well hope to hear from you soon

Answer
Dear Lou,
Thank you for sharing your long winded aquarium story with me. I wish I could have gotten my newts to live. They were the most vulnerable fish I ever had. The first time they kept crawling out and I'd find them dead on the rug, or where ever. The second time, my plecto killed them off one by one.
Now I'm being long winded. The upshot is that your tank is full. If anybody kills anyone, it's a definite sign that space is getting tight and they are feeling crowded. Anybody can get along in a tank if there's plenty of room (except two betas, of course), but nobody will get along with anybody if it's too crowded.
I don't want to recommend that you get another tank, because then you would not have time to do anything else in your life, such as go to work or school, eat, or sleep. So I'm going to recommend that you stick with what you have until the situation changes. You are the type who probably wants a pond. God help you. He he.
I don't know of any web sites, but if you find some good ones, send them to me and I will add them to my page for aquarium care.
http://steamboats.com/aquarium.html
Hope this helps,
Sincerely,
Nori