Hey Nick. Welcome to the hobby/addiction!! It sounds like you've got a pretty good understanding of the basics. Being mindful of your bioload, from the beginning is a great way to start.
Filtration should be adequate, if the system was running and healthy. Although don't know the sizes of the Penguins.
A good deal is all perspective, there are some steals on CL
, if your lucky enough to find them. All together the setup sounds like a good start.
Be careful not to over feed, every day + treats is a little much, but OK if it is only a very small amount. Excess fish food is the leading cause of poor water quality, which then leads to algea outbreaks. Especially without a skimmer, I would watch your feedings.
As far as more stock, IMO
you could add a few more smaller fish, 1 at a time preferably, like firefishes, gobies, or blennies, or smaller Wrasses. Not totally sure about these fish with a Valentini Puffer, but you can easily check for compatability. Clowns and Chromis would be fine with any of these though.
I believe a Valentini will harass clams and some inverts, though your lighting will not support clams unless very strong and it sounds like your lights are mainly for fish, but be wary of inverts with the puffer.
Anyway hope this helps, and have fun!