OOP with MVC using Mongoose & Express.js

I'm creating an Express.js app in which I want to use the MVC pattern and Mongoose for the document mapping to a MongoDB database. I've created a folder for models and I want to derive everything from (Javascript's version of) abstract classes for better code organization.

I'm confused about what the best way is to organize the abstract classes and set default values that each instance of the models should be. For example, one way is to use Mongoose Schemas for abstract classes, and then use Mongoose models for the models themselves:

Feline.js:

var mongoose = require('mongoose');

var Feline = mongoose.Schema({
  size: 'Number'
});

Feline.methods.getSize = function () {
  return this.size;
}

module.exports = Feline;

HouseCat.js:

var mongoose = require('mongoose')
, FelineSchema = require('./Feline.js');

var HouseCatModel = mongoose.model('HouseCat', FelineSchema)
, HouseCat = new HouseCatModel({
  size: 1 //Domesticated cats are small
});

module.exports = HouseCat;

There are a few problems with this design. For one, I would think there must be a better way to set specific properties for the each model without instantiating a new model object each time the client wants to create a new instance of a model type. For another, using this scheme, Mongoose has to be required in every model file, and the code is custom-tailored to use mongoose, which means it will be difficult to switch to another ODM if we want to do that in the future.

Is there any better way of coding this? And is there any design pattern which is easy enough to implement in Node that will allow for easy changing of the ODM?