If I know there is only one God and all are his forms, Can I worship(at temples) one avatar of His' with mantras of His another avatar/form (among Shiva, Shakti, Ganapathi, Skanda, Surya, Vishnu and Agni)?
Well even though it is a fact that God is one singular power(or shall we say, "energy") there are different form to it. Just like different kinds of energy, these various forms are invoked using different mantras. (A human cannot gain energy from fire or water directly. He needs to eat). Every mantra is made up of Bijāksharas and each of them correspond to different forms of what we call, God. Unlike our western counterparts who hold the viewpoint that God is always good, even god can have relatively positive or negative forms. That is why we have Kali/Jyeshta/Alakshmi(goddess of misfortune) and Kāli which is a non-peaceful form of Shakti.
Hence, it is better to stick on to using a mantra meant for worshipping one form to be used exclusively for that purpose. Wrong pronunciation of mantras(to be specific, Bijāksharas) may lead to the invocation of some powerful forms requiring special rituals to be kept at check. If not, the aura of the deity may bring upon mishaps.