Feeding treats from your hand is a great way to make friends! I don't think I'd put on the harness at this point though, that would probably be kind of upsetting to him and you're trying to build trust.

A cockatiel seed mix would probably work well for him. Seed mixes and pellet sizes are really based on a bird's beak size more than anything else, so go by what he can actually eat not the species that's listed on the label. His body and beak size are fairly similar to a cockatiel's so cockatiel-sized food should work well for him. If he can handle Tiko's food then that would work well for him too. He needs vegetables too of course like all parrots, and apparently this species eats a lot of fruit in the wild so give him some of that too.

Plumheads are classified with the Asiatic parrots, so it might be helpful to look for a group that knows a lot about Asiatics. Psittacula is another name you could search for, that's the genus that includes Indian ringnecks and plumheads and several other related species. You probably aren't going to find a forum that specializes in plumheads, but you might be able to find one that specializes in Asiatics/psittacula. Plumheads are closely related to Indian ringnecks, so a group with a lot of IRN owners might be helpful if you can't find anything else.

His beak must look bigger in real life than it does in pictures! Anyway, feed him the kind of food that he can handle. Some sites say to give them a small parrot mix so you could try that.

Plumheads are in the same genus with IRNs, moustached keets, and Derbyans, so they all have a lot in common.

In cockatiels, patchy foot coloring is usually an indicator that the pied gene is present (either visual pied or split pied). I wasn't sure whether plumhead mutations actually existed, but I did a search and found this site: http://psittacula-world.com/EN/Mutation ... ephala.htm They have more mutations than I expected, including a recessive pied mutation similar to what cockatiels have. So it's possible that he's split to pied.

To me the foot problem looks more like an amputation than a birth defect. The foot looks normal, except that part of the front toes is missing. The "cutoff" line is nice and straight. In humans, an amputation usually looks like this. With a birth defect it looks different - the affected body part doesn't look this normal, and it tends to sort of taper off in an irregular way. But it doesn't change anything no matter which one it is, so it doesn't really matter.