You can try out yourself whether the games lags or not, can you? There is no ultimate guideline that holds everywhere, really. It depends on a number of factors such as how many objects of that type there will be in a typical scenario and how many polys you actually need for the model to look halfway good.

As a developer I don't want to give you rigid constraints on how the models should look like. You are a creative person, right? This is not like in a business relationship where the customer dictates all the facts. A bit of initiative is required by everybody involved. In any case the modeling process will likely take a few iterations where feedback from the community is fed back to the modeler. And a model that is not optimal is still better than no model and should be used in the game until someone (yourself or another modeler) is annoyed enough to improve the model (using the existing model as a basis or starting from scratch).