1. Going for Hajj with Sunni Muslims does not harm your Hajj as far as you perform the Hajj in the proper way according to the Prophet (SAWA) and Ahlul Bayt (AS). You need to read a book of Hajj steps according to Ahlul Bayt (AS) or to contact Shi'a scholars how can guide you.
2. Going for Hajj on behalf of a Sunni deceased person is allowed if you have already performed you own obligatory Hajj, or if Hajj is not yet obligatory on you because you can't afford it.