I'm not sure what the gph rating is on the Penguin 330....is it 330? If so I might look into getting an additional filter if you're going to add more fish. If thats not a possiblity then you'll still probably be ok....just I would think about it.
I think as long as you keep up with maintenance and the filtration is adequete then you should be fine adding the fish you mentioned. Personally I would add a few more guppies and a few more mollies and maybe a couple of ottos...but sounds like you got it figured out.
Don't forget to add the fish slowly! like one or two every week or two so that the biological filter can catch up with the extra load.