Is this fine in Magic: The Gathering?

I'm trying to make a new deck for magic the gathering, and I don't have many cards. I have found 5 things I don't understand while making it and trying to save money by not buying new cards. Does the card Utopia Sprawl, with the effect "Enchant Forest, As Utopia Sprawl comes into play, choose a color, Whenever enchanted Forest is tapped for mana, its controller adds one mana if the chosen color to his or her mana pool," work with Snow-Covered Forest, which is Basic Snow Land - Forest? And with Sea's Claim, "Enchanted land is an island," does the enchanted land now give blue mana, or does it still give whatever colour it would have before? And when a card says that by tapping it you add 1 mana to you mana pool, does it give a colorless mana, or any of your choice (examples "Rix Maadi, Dungeon Palace", "Prahv, Spires of Order", "Guardian Idol", and "Prismatic Lens")? And if it gives colorless mana, does colorless mana work as any many, or only for paying colorless mana costs, like with most Myrs? Finally, is 72-74 cards too much, not by recommendations, but by rulings?