I guess my question is how to fixture one component and apply a force to another?
It's somewhat tricky. All finite element models (minus non-linear dynamics) require that all components in your model are constrained against rigid body motion, which can be somewhat confusing to people. In short, you need to make sure that you have enough constraints defined such that your parts can't freely move as a rigid body in space. My guess is that the top pipe in your model is under-constrained; how you properly constrain it depends on system and how it function, so I'd need to know more to be able to tell you. However, the solution will most likely involve soft-springs to stabilize the model.
Out of curiosity, is your entire goal to calculate the force required to insert one pipe into the other? Do you expect the lower pipe's outer surface to not expand in the radial direction (I ask because it looks like you have a constraint that prevents radial expansion of the outer surface of the lower pipe)? Do you need any information (displacement, stress, etc.) on the lower plate? If so, how exactly is the lower plate constrained?
I ask all of these questions because, based on what I can see, the information you've provided, and the assumptions I'm making, I think you can do this simulation with a 2D axisymmetric idealization. If this is the case, the the simulation time required for this model can easily be dropped by an order of magnitude or more.
