Do signed letter valid for any legal actions???

Respected Sir/Madam I was working in a proprietor firm for past 5.5 years. They were doing a trading business. It was registered company VAT, CST and IEC. During my service they have not given me any appointment letter, salary slip, esi, pf or no mediclaim. but they credit salary in my bank account for all past years. I helped one of my customer by giving a supplier name to buy parts as my company stopped giving service to them due to some payment issues. Also as they we abroad they need some parts from INDIA i purchased in delhi and couried them for which they made me payment also. When this matter come to my company notice, immediate they forced me to resign job and took a letter from me stating i will not engage in similar business activities nor contact any of their supplier and customer. Is my given signed eligible for any legal action?? i signed letter due to their compulsion. Now can i start my own business in the similar field i was working for them?? Can i buy product from the same supplier or sell to same customer to whom my ex-company do?? kindly clear my above doubts?? thanks samy