Ally Triggers

When Oran Reef Survivalist ( http://gatherer.wizards.com/Pages/Card/Details.aspx?multiverseid=178138) enters the battlefield doesn't his +1/1 buff use the stack meaning he could be pinged off by anything that could do one damage until his trigger resolve

Same with (http://gatherer.wizards.com/Pages/Card/Details.aspx?multiverseid=197403) he comes in as a 2/2 with a 1+1 counter on the stack so he's still susceptible to a top stacked disfigure

Also if two allies enter the battlefield (say a cleric and a survivalist) at the same time, which ability is top stacked? The life gain or the 1/1 counter?

If to creatures entered the battlefield under your control at the same time you could choose which order you wanted their abilities to go on the stack.

"Enters the battlefield" is a triggered ability.

Quote :

603.1. Triggered abilities have a trigger condition and an effect. They are written as “[Trigger condition], [effect],” and begin with the word “when,” “whenever,” or “at.” They can also be expressed as “[When/Whenever/At] [trigger event], [effect].”

Triggered abilities use the stack

Quote :

603.3. Once an ability has triggered, its controller puts it on the stack as an object that’s not a card the next time a player would receive priority. See rule 115, “Timing and Priority.” The ability becomes the topmost object on the stack. It has the text of the ability that created it, and no other characteristics. It remains on the stack until it’s countered, it resolves, or an effect moves it elsewhere.

If more than one ability triggers at the same time, the controller of those abilities adds them to the stack in any order of his choosing.

Quote :

603.3b If multiple abilities have triggered since the last time a player received priority, each player, in APNAP order, puts triggered abilities he or she controls on the stack in any order he or she chooses. (See rule 101.4.) Then the game once again checks for and resolve state-based actions until none are performed, then abilities that triggered during this process go on the stack. This process repeats until no new state-based actions are performed and no abilities trigger. Then the appropriate player gets priority.

APNAP is an acronym for Active Player, Nonactive Player (APNAP). The active player is the player whose turn it is. The active player chooses the order his spells/abilities go on the stack first.

Quote :

101.4. If multiple players would make choices and/or take actions at the same time, the active player (the player whose turn it is) makes any choices required, then the next player in turn order (usually the player seated to the active player’s left) makes any choices required, followed by the remaining nonactive players in turn order. Then the actions happen simultaneously. This rule is often referred to as the “Active Player, Nonactive Player (APNAP) order” rule.
Example: A card reads “Each player sacrifices a creature.” First, the active player chooses a creature he or she controls. Then each of the nonactive players chooses a creature he or she controls. Then all creatures are sacrificed simultaneously.

Another relevant use of triggered abilities and the stack are landfall abilities.

Landfall is a triggered ability and uses the stack. Playing a land does not use the stack. Using a fetch land to search for a basic land is an activated ability and uses the stack.

Q: When can I use burst lightning to kill a steppe lynx?

A: In response to a land being played. Since the landfall ability goes on the stack, you can add an instant to the stack that would kill the lynx before the triggered ability resolves. However, if the land was a fetch land its controller could then use the activated ability adding another landfall trigger to the stack that would resolve before the burst lightning and save the lynx.