No way to insert a custom serializer inside a model serializer?

channel.py

from rest_framework import serializers
from rest_framework.serializers import ModelSerializer, Serializer

from api.models import Subscriber
from api.models.channel import Channel
from api.util.connection import get_redis_connection
from api.util.field import ReverseField


class ChannelStatsSerializer(Serializer):
	"""
	Serializer for embedding channel stats in channel object.
	"""
	subscriber_count = serializers.SerializerMethodField('get_subscriber_count')
	post_count = serializers.SerializerMethodField('get_post_count')

	def get_subscriber_count(self, channel):
		redis_server = get_redis_connection()
		return redis_server.llen('subscriber:{}'.format(channel.id))

	def get_post_count(self, channel):
		redis_server = get_redis_connection()
		return redis_server.llen('channelfeed:{}'.format(channel.id))


class ChannelSerializer(ModelSerializer):
	"""
	Serializer for retrieving and updating channel information.
	"""
	url = ReverseField(view_name='channel-instance')
	posts_url = ReverseField(view_name='channel-post-list')
	privileges_url = ReverseField(view_name='channel-privilege-list')
	subscribers_url = ReverseField(view_name='channel-subscriber-list')
	stats = ChannelStatsSerializer(read_only=True)
	current_user_is_subscribed = serializers.SerializerMethodField('is_current_user_subscribed')

	class Meta():
		model = Channel

		read_only_fields = ('date_created', )
		non_native_fields = ('stats', )
		exclude = ('subscribers', )

	def is_current_user_subscribed(self, channel):
		"""
		Check if the current logged in user is subscribed to this channel or not.
		"""
		request = self.context.get('request', None)
		user = request.user

		if user.is_authenticated():
			return Subscriber.objects.filter(user=user, channel=channel).exists()
		else:
			return False