The limit is 4s I believe, but in reality if you can avoid using it completely it saves a little extra heat & work for the esc to do. If you are concerned about the size of the ubec (the RCM one isnt all that small, but it is flat), the
Dimension engineering one is smaller than a gnats testicle, so it will be easy to fit just about anywhere on that stunning truck of yours- Mike has done a cracking job there.