Sunburst (This enters the battlefield with a +1/+1 counter on it for each color of mana spent to cast it.)
Remove two +1/+1 counters from Spinal Parasite: Remove a counter from target permanent.
Any permanent is a legal target for Spinal Parasite’s ability, not just one with a counter on it. The player who controls Spinal Parasite chooses which counter to remove from the targeted permanent.
If Spinal Parasite is a 1/1 because it has two +1/+1 counters on it, you can remove both counters to pay for its activated ability.
Yes, this card’s base power/toughness is -1/-1. If it has five +1/+1 counters on it, it’s a 4/4 creature.