In the end Amiibo are just AI just like the CPUs in game.
You may be able to feed them and teach them to be offensive/defensive but they're really not much different. A lot of them still won't take advantage of certain things, like recovering with a Side B move or do something a human player would do (Ganoncide, using Links bombs to help recover, etc). They really wouldn't serve much purpose of helping you improve because of this.
On top of that, a lot of them even have some pretty bad quirks. Some Amiibo will spam certain moves no matter what or refuse to use a certain one. Villager for example has never been known to use his tree or even axe. It can't get past the sapling state. Pac-Man is another example too. While you can get him to use his Hydrant he ignores it completely like it's not even there until you hit it. Pac-Man has also been known to have difficulty using his fruit effectively. It's hard to get him to charge past Cherry.
In short, some Amiibo are just pathetic AI wise while some have pretty good AI for the most part. (Like Shulk)
I've read a lot of Amiibo guides in the past and own just about every single Smash Amiibo except for three (MetaKnight, Wii Fit Trainer, and Rosalina). I spent time training every single one and doing what I can to make them perfect minions of pain.
Bottom line is I think the only use you will really get out of them is the enjoyment of training them and it's something extra.
As for feeding them or keep them vanilla, that's entirely up to you. They still get a buff whether you feed them or not and I believe I read somewhere they learn more effectively if you do feed them. The most important thing to consider in my opinion would be what custom moves they can benefit from the most. Also keep in mind that the more an attack lands the more they'll use it.
I will say feeding them might be important if you want them to go against other Amiibo, but it depends I suppose.
There's a site called
"Amiibo Dojo" that's pretty helpful when it comes to training them. It has a guide for just about every existing Amiibo currently that's well typed out and gives you a lot of tips about each specific one, so use that if you want.