Last year a new zone for in floor radiant heat to a new kitchen was added to an existing boiler. (Weil-McClain CGi8) The existing loop heats 100 year old hot water radiators and had been working fine for years. The circulator pump for the new zone is on the supply side, between the mixing valve and the supply manifold to the 3 pex loops. The circulator for the existing loop is on the return side. The new loop T's in to the old loop just downstream of the circulator pump outlet. This just seems wrong - won't it cause the pressure to be higher on the return side of the new loop, or make the pumps fight each other? Any thought would be appreciated.

It would be easier to understand with a drawing. It doesn't sound correct, and if it's done the way I'm thinking, you may be right about the circulators fighting each other. It could be made to work though with proper placement of check valves and use of a switching relay with priority zone. You would typically make the radiators the priority zone, as they would cool much quicker than the radiant. So, with priority, only one zone would ever run at a time.
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I hope they incorporated some type of boiler protection with the addition of radiant to that boiler. That is a cast iron boiler, and low return temperatures from the radiant will cause flue condensation, and rot that boiler out in short order.
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Edited 3 times.

it doesn't sound right to me either.
you need to have some sort of tempering valve in there so you don't run 180 deg water into the floor heat. there are diagrams with the tempering valve that will tell you where and where not to locate the circulator.
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can you post a picture?

The OP states that there is a mixing valve on the radiant loop.