If you modify unit statistics (so certainly "combining" multiple units in one), it will create a mod that can only be used in multiplayer if the opponent has the same mod. And of course, this mode will have to be balanced from scratch again, including all bugs that accompany balancing.
However, it should be possible to improve the performance a lot by using simple models and textures. F.e. turn every tree into a green, tall pyramid; remove all details from buildings and bring them down to the essence (blocky fortress, house with a few tris, ...). It should be able to use this mod to play against someone who plays the main game (with beautiful graphics), but such a low-quality mod should still have some serious performance benefits.