Actually, territoriality associated with a cage does start when the bird is sexually mature. Babies are not territorial.
My goodness! There is still people going to newyorkbirds? I was one of the three original people that started that birdsite years and years ago and, most likely, what you read in it about gloop was all posted by me -LOL- as I was the 'gloop lady' in it! Linda didn't even feed gloop but then she was old-fashioned and did not even followed a solar schedule (she was still doing the 12L/12D one). But you will only find the old recipe in there so check the diet section here for an updated one (I am always tweaking it and the recipe has changed a lot since then).
Now, first of all, you have to make sure that you feed organic the stuff that needs to be organic (bell peppers do) so check the annual list for the 12 dirtiest veggies and fruits and always buy those organic. This is the site that gives you the list:
http://www.ewg.org/foodnews/ Secondly, I have found that it's harder to get them to eat veggies if you serve them by themselves BUT they all end up eating them when you mix them in the gloop (for lovies, you need to make sure they are finely chopped or of a small size naturally -like corn and peas, for example). Start with a grains and pulses only gloop and mix a bit of seeds in it and, as you see he is eating the grains and pulses, eliminate the seeds and start adding veggies one by one (start with fresh sweet corn, they all LOVE it!). Lovies are partial granivores so feeding pellets really makes no sense whatsoever. The only thing is that you need to supplement vit D3 so make sure you are giving him some vitamins, too (I use a one and half to two times the recommended daily dosage once a week).