I was looking on ebay for cards, and came up with a control deck, if I"m reading the cards right. I don't know if it's already been discussed, but I hadn't found it on the rules pages yet.

Monstro lvl 1 and Dark Riku: Does it reduce the number of dark cards in my field down to 3, or can I have as many as I want in my friends area because they're dark, not friends?

The way Dark Riku works is the dark cards that you play in your friend area are considered to be both 'friend' 'n 'dark' cards (or, in the rest of this paragraph, I call them 'dark/friends'). Meanin under the effects of Monstro, you would be limited to three 'dark/friend' cards. Also noted, if your opponent casts any magic cards, they can very well target the 'dark/friend' cards on your side of the field (as a way of destroyihn the cards that would prevent them from bein able to play certain cards {Parasite Cage, Sea Neon, etc}). Don't let this discourage you, however, as dark decks are considerably fun to play around with (so many combos to pull off! Where to start?). But certainly, Monstro would limit you to only havin 3 'dark/friend' cards on the field.

Your dark cards are considered your friends. They are cut down to 3 at a time as well.

You can however play a 4th and discard another to reduce to three again.