RCM AGAINST COMPOSITION

If you buy goods from dealer registered under composition scheme then RCM will not be applicable as he is not unregistered dealer

No you don't have charge GST under RCM basis. Since he is register person and RCM will be applicable if purchase from unregistered supplier. If you want to clarify it then you can read section 9.

[4] The central tax in respect of the supply of taxable goods or services or both by a supplier, who is not registered, to a registered person shall be paid by such person on reverse charge basis as the recipient and all the provisions of this Act shall apply to such recipient as if he is the person liable for paying the tax in relation to the supply of such goods or services or both. section 9 (4)

RCM applicable if registered person buy goods or supply or service or both from a unregistered person as per sec 9(4) of cgst 2017