What's the best PrefixStyle with ProtoBuf-net for big files?

I need to store big data (in the order of Giga bytes) to stream using protobuf-net 2.4.0.

Currently, I use the strategy of writing a small header with LengthPrefix with PrefixStyle.Base128 and the big body with the standard protobuf serialization approach with the below code, and it works like a charm.

private void Serialize(Stream stream)
{
    Model.SerializeWithLengthPrefix(stream, FileHeader, typeof(FileHeader), PrefixStyle.Base128, 1);

    if (FileHeader.SerializationMode == serializationType.Compressed)
    {                
        using (var gzip = new GZipStream(stream, CompressionMode.Compress, true))
        using (var bs = new BufferedStream(gzip, GZIP_BUFFER_SIZE))
        {
            Model.Serialize(bs, FileBody);
        }
    }
    else
        Model.Serialize(stream, FileBody);
}

Now I need to split the body into 2 different objects, so I have to use the LengthPrefix approach for them too, but I don't know what is the best PrefixStyle in such a scenario. Can I continue to use Base128? What does "useful for compatibility" mean in Fixed32 description?