Can I train to make heavy armour once I have gone down the light armour route?

I've started doing dwarven ruins and have noticed how much money and skill I could make with all the scrap.

Yes you could do both routes if you wanted too. All it's going to take is a couple extra perks that you wouldn't put in another skill.

Yeah, you can fill the entire tree if you want.
You ought to spend perks on something that'll make your character more oomph though, as you'll be swimming in septims anyway.

If you go the light armour route you might as well go all the way. You'll go from steel to steel plate (as good as orcish and at the same level) to dragon plate. It's some big jumps, but the ingredients for those armours are all quite common. Elven and glass weaponry is quite a bit better than dwarven and orcish weaponry too.