(I'm new I don't know where this thread belongs)
Typhlosion has Eruption which is a 150 damage SP. fire attack, so,
Does that move make Cyndaquil a good starter compared to Other fires?
I feel like that move can one shot everything if I EV train SP. Attack.
How is Typhlosion in competitive?
Typhlosion is in RU, to me that means that it's not that good because I mainly play OU. I don't know how it does in RU so you're going to have to wait for someone else to answer that.
As PlatinumDude said, Eruption is best when you're full health. At 75% health (after one Stealth Rock switch-in), it's getting ~112.5 base power, which is just a bit more than Fire Blast. But if you get lower than that, then Fire Blast is better, and there are much better choices than Typhlosion for a strong Fire Blast.
Typhlosion's movepool is also terrible. It gets Fire moves, Focus Blast, and Hidden Power (on the special side). Its offensive stats are low compared to other Pokémon (of course, I'm making the comparison to OU Pokémon, not RU). A lot of Pokémon can tank one Eruption and KO back or deal enough damage to the point where Eruption is irrelevant. Yes, Specs Eruption hurts and is capable of winning, but there are a lot of faster Pokémon in RU, UU, and OU that can deal with Typhlosion very easily. You have to predict the switch-ins if your opponent plays like that. But usually they'll just sack a Pokémon and then come and revenge kill you.
In comparison to other fire starters, Typhlosion is the worst. All of the other Starters are either broken (Blaziken) or have different options that they can go for. Typhlosion's bad stats and terrible movepool doesn't give it many options (the only mix-ups you can have are between Scarf or Specs, or maybe Life Orb and then choosing what Hidden Power type to go with). I guess you could also run a physical set, but that's just cheese.