Pretty nice set of initial pens, Dominic. Be sure to save some of them to look at a year or so from now & see how much you've progressed.
Nothing at all wrong with making "slim" slimlines - I make some like that once in a while too, but I generally prefer leaving a little more meat on the barrels without making too much of a wasp-waisted center. I sometimes use a larger centerband - like the ones from a comfort kit or make my own so I can make a little larger pen with nice proportions. The slimline gives you a lot of flexibility for creativity & variation - don't be afraid to experiment.
Have fun - looks like you are off to a great start!
Edgar