First, I would like to thank all people which tried to help me with their answers.

1. Second, I have to point out how sad the fact is that all pet shops (not 1, not 2 but 5) I consulted for the size of my tank told me that 20 liters (5 gallon) are sufficient for hosting 4 to 5 goldfish. I am generally sick of what people will do or say for a few pieces of paper (called money). I strongly consider to start building my own 125 liter (30 gallon) tank -> suggestions, tips ?
2. Finally, regarding my initial question, I had to separate my Betta because I noticed some scales missing on the poor Black Moor ! Result is that the 3 goldfish are happy and playing all over the tank, but the Betta looks sad (or bored ?) without company.
3. PS : I really really checked it out and besides the Black Moor, the Oranda Calico (white body and fins, double tail fin and redish covered gill area) the other is the COMMON goldfish (goldish colour all over body and fins, single tail fin) and NOT a Comet.
4. 5 gallon for 4-5 goldfish is WAY too small. Goldfish poop alot and the water gets dirty very fast. Moreover, with 4-5 goldfish in a tank as small as 5 gallon, the water is bound to get dirty very quickly leading to many diseases.
   For your 30gallon, I'd suggest you read up on fish characteristics first. I always suggest Zebra Danios for beginner fish hobbyists. They are hardy and they tolerate a variety of water conditions. It is better to keep them in schools of 5 or more. You are going to need a filter, you are going to need an air pump, you are going to need a heater (not unless you decide to have only goldfish, theyre coldwater.But variety is always fun) But READ first.
   I wouldn't worry about your betta as long as he is eating well and swimming normally.He might looked bored to you cos you've seen him in action for the past couple of days. But I don't think it is anything to worry about.
5. Yeah, it is sad that clerks in tropical fish stores 9 out of 10 times don't have a clue what they are talking about. You're better off doing your research on the web before you go buy your fish (I have tons of printouts from the web on things I am looking into). Goldfish grow pretty fast and produce a lot of waste so they need a whole lot of tank space. My local fish store has black moors and orandas that are about 5" long (not counting their tails) and I don't think those are full grown yet. Also, goldfish need cool water (20C degrees) but bettas need warm water (26.6C degrees) so you shouldn't keep them in the same tank. A young betta will do all right in room temperature water (if you don't keep your rooms too cool) but as it gets older, it'll have a harder time staying warm and will need a warmer tank or it will begin to suffer a lot of illness.

   You're betta is fine. Male bettas spend a lot of time all alone since they don't get along with most other fish. In the wild they'd spend most of their time hunting for food. When you're home, spend as much time with him as possible (they do have personalities and will respond to you), for very short periods of time, put a mirror against the side of his tank so he can see his reflection–he'll think it's a strange beautiful male fish come to challenge him and he'll flare (don't do this all the time or for long stretches since he will release hormones which will collect in the tank, and flaring raises his stress levels). You might be able to push his tank up against the goldfish tank so he can keep and eye on his former roommates.

6. Bettas often attack other fish with flowing fins, and I'll bet your moor's fantail was just too much for him.

   I've found that bettas get along fine with cory catfish. The corys stay low, the betta stays high, everyone is happy.

   Also, bettas seem to like having some foliage in the tank to swim around and maybe some decor to interact with.

   If he's bored, you may laugh, but you can train him. Check this out: http://fish-school.com/

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