Q. 2. In olden days as far as I know people used to worship Lord Sri Krishna only. I think it was in our (ISKCON) spiritual line Sri Sri Radha Krishna are worshiped?

                                                                  You have asked a very interesting question, regarding the introduction of Radha and Krishna worship, as opposed to worship of Lord Sri Krishna without Srimati Radharani.

                                                                  For your information, Radha Krishna worship is also found in the Nimbarka Sampradaya as well. The Vraja dham affection of conjugal love between Radha and Krishna and the very confidential mood of service found in Vraja in general is enhanced by the worship of Radha Krishna. In fact, it is by the mercy of Srimati Radharani, and the other associates of Vrindavan, that one can enter into the confidential understandings of the love of Krishna that is known in Vrindavan only. Worship of Krishna without His hladini sakti, Srimati Radharani, which is found in many places in India, not only in the past but even at present, permits one to enter into the aisvarya mood of worshipping the Supreme Lord in opulence. It is understood that some parts of the paraphernalia of Lord Krishna represents the internal potency of the Lord. For example, in the Vallabha sampradaya the mala of Sri Nathji is understood to be the manifestation of the Lord's internal potency, or Srimati Radharani. In this way, worship is also performed, but in a mood of opulence rather than in a mood of intimacy, as the worship of Radha Krishna facilitates.

                                                                  Q. 2. In almost all avatars it has been that Lakshmi is the consort of the Lord. For ex: Whether be it mother Sita or Visnupriya or Padmavathi (Lord Venkateswara's consort). And not Radharani., why is it that way?

                                                                  I think you have a fundamental misunderstanding here. As Krishna is the original personality of Godhead, so Srimati Radharani is His hladini sakti manifestation, or His eternal consort. Since Narayana in Vaikuntha is an expansion of Krishna, we can say that Lakshmi is also an expansion of Srimati Radharani. For that matter, all the goddesses of fortune in the spiritual realm are manifestations of Srimati Radharani. Your thinking seems to be in the converse. Having made the above statements, we can identify the particular consort of the Supreme Lord by a particular nomenclature also. For example Sita is the consort of Ram. Ramadevi (found in Sri Brahma Samhita) is the consort of Maha Visnu. In your letter you have indicated that Padmavati is the consort of Lord Venkatesvara. Each of the expansions of the Supreme Lord have their consorts, or energetic expansions. They go by various names, but the original Personality of Godhead is Krishna, and His eternal consort is Srimati Radharani.