Hi :-)

1) Yes, multiple colours can strengthen the weaknesses inherent in a single colour, and allow you access to things not available in that colour. The downside is making it more difficult to get access to the right coloured mana at the right time. At the moment in Standard, though, the manabases are really good so lots of people are playing three or four colours, as it gives access to some very powerful cards.

2) Aggro decks usually don't have any main-board creatures or spells that cost more than 4 mana. Most don't even go above three. mtgtop8 has a ton of decklists and classifies decks by type. Have a look and see what other people are doing.

3) The only way to test a deck is to play. There are online options (MTGO, xmage) and paper options if you have an LGS nearby. Or, the old standby of "goldfishing" and playing against yourself.

4) I don't see what the benefit is of the soldier theme. There aren't any cards that care about how many soldiers you have or that only buff soldiers, so why limit yourself to only sodiers? Even if you'd still like to do that for flavour reasons, there are some excellent red soldiers, and red/white aggro is a classic aggro colour pairing. One thing white lacks is being able to aim damage at your opponent. Red has spells that hit creatures or players that can be used to get that last bit of damage in to kill your opponent.

5) The best defense for Aggro is to kill your opponent before they win. There's not a lot of cheap removal or cheap counterspells around at the moment. If you play a one drop on turn one, and a two drop on turn two, and a one-drop and a two-drop on turn three, then you're well on your way to killing them on/by turn five.

6) Absolutely worth it. Even if you can't protect him, he's a massive threat to your opponent that they can't ignore. I would play every copy I have.