1:  Whether a person can avail the composition scheme on Small Retail Trading of goods if he is holding both incomes like Sale of business: Rs.25 lakh (Small Retail Trader) and Rental income: Rs.12lakhs, whereas the person was registered earlier in VAT Composition Scheme and was paying Service Tax on rental income?

Renting is a service and supplier of service, except restaurant service, cannot opt for composition scheme. Since you are supplying both goods & services, you are not eligible for composition scheme. 

2:  Can traders selling on ecommerce portals avail composition scheme if their turnover is less than 75 lakhs?

No, Sub-section (2) of section 10 refers.