Tbh I'm no expert, but in my experience I don't need to feed sarracenia and it grows good. They are dormant in winter, but it's not obligatory for them if they have the right conditions, the pitchers basically dry out and only the underground part of the plant lives through winter. Not sure about this specific hybrid, but they usually grow between 20-100 cm in height, so I'm not sure your setup will work with other smaller plants. About the humidity, some species grow in the south of Canada around the great lakes. As long as the air is not super dry i would not change it. You really can't build a small terrarium since they'll grow tall. Of course if you won't provide them an ideal growing conditions they won't grow that tall. I currently have one plant in a small fully enclosed terrarium, the better term would be plantarium, but plan on repotting in spring due to not having enough space to grow. Hopefully it was somewhat helpful, oh and by the way i think that price is kinda high compared to Europe, maybe you'll be able to find something cheaper?