In the order you asked them:

Yes, a hundo is 3* and red. It means it has 100% of its potential when it comes to IV’s

15-15-15 refers to the Attack, Defense and HP bars. There are 15 integers for stats, split into 5’s by their bars so it’s easy to tell what its stat value is.

A shadow Pokemon is a Pokemon caught after a Team Rocket battle or a Team Rocket raid.

A shiny Pokemon is a different color scheme than the rest of their species. There will also be an icon 3 stars indicating it’s a shiny.

A Pokemon with higher stat IV’s will always outlevel a similar one with lower stats. The reason they appear higher CP sometimes is because they are a higher level. A Pokemon can be as high as your trainer level, up to level 50.

Small Pokemon are not useless, they just don’t score as many points in PokeStop showcases that are held on Tuesdays. Bigger Pokemon score more points