Selling a variety of brands won't effect any chance you have of forming a partnership with a brand owner in the future, however you won't require a partnership with any brand owner who's products are openly available through a wholesale market.
The only time you will need to form somewhat of a partnership to sell a branded product would be in an instance of the brand owners product's only being sold through an authorised network.
These are mostly designer brands that will require this, and becoming an exclusive distributor, if even possible, will not be an easy accomplishment by any stretch of the imagination.
Some designer brands will only be sold through their own stores, they won't offer any distribution outside of that at all, and that is to protect the integrity of the product and the brand name, and therefore control the product price.
Those that will will distribute to exclusive retailers will make some very high demands, and you would need to establish a proven record of trading with the uppermost integrity to even get a foot in the door, not to mention a large bank account for any licensing rights.
I think sticking to regular wholesale available branded products is the way for you to go, once you're up and running you will be in a better position to know where you want to take the business in the longer term.