Might be stressed. Are there heat and/or pest issues? Could be from many things. Might just be the plant, some stay pretty small. Switch em to flowering if yo want, they will def grow
I use the one from homedepot for like 9 bucks. Workts well but not light proof. I imagine you could put some panda over it but my tents in my closet anyways so no worries
Umm if by veggies you mean tomatoes sure, but not cannabis. You need atleast two 600 watters to cover that area unless your just going to veg them there but that still seems like insufficient light for the size