I am not sure I understand all of your questions, but some of them depend on your broker:
1. buying/selling the same pair or CFD at the same time depends on if your broker follows the US trading rules that does not allow "hedging." The USA has stricter trading rules and if your broker is in the USA then you will not be able to buy and sell the same pair at the same time. I use FXCM UK and not FXCM US for this reason.
2. Martingale is a long discussion topic but it has a lot of risk because if you have several losing trades in a row the system will multiply your positions quickly and you will use a lot of margin and it is much more likely you will have a margin call or a forced closing from your broker. Try this on a DEMO account to see what I mean. There are always discussions on this Forum on martingale.
3. The number and size of your trades has a limit depending on your account size but you should always only use a small part of your account otherwise you can get margin calls quickly and be forced out of the market. Some brokers have minimum order sizes but most will not. Forex brokers usually don't care about scalping, which usually involves a larger amount of small trades in and out quickly. The broker will tell you if they allow or limit scalping.
I don't know what you mean about "knowing the trade balance is banned."
Good luck. TRADE with a DEMO to understand these basic points in real life.