Short answer: no, you don't have to worry. Thanks to our CSS partnership (Comparison Shopping Services) with Google, Tanganica campaigns run alongside your existing campaigns, whether you manage them yourself or through an agency, without hurting them. In most cases they actually help.
How Google handles it
Google treats all offers for your shop as one merchant, no matter which CSS uploads them. If two campaigns enter the same auction with the identical product, Google shows your offer only once, so your ads never compete against each other for the same spot.
You are never second priced against yourself. The price of a winning click is always set by the next competing shop, not by your other campaign. Running two systems therefore does not increase your CPC. (Source: Google Merchant Center Help, Advertising with Comparison Shopping Services.)
Why run Tanganica Performance Max next to your existing campaigns
A second independent bid. Every bidding system decides on its own whether and how much to bid. Auctions your current campaign skips, the Tanganica system can enter, so your shop takes part in more auctions overall.
Coverage of a different part of your catalog. Two systems with different signals and priorities serve a different product mix, even on the same feed. The second system typically picks up long tail products the first one leaves behind.
A live benchmark. You get an ongoing comparison of two independent systems over the same assortment, which keeps both sides motivated to perform.
Backup. Two feeds and two accounts mean that if one side runs into a feed or account issue, your Shopping presence doesn't go dark.