This is NOT a Chihuahua. It is a Sphynx cat. A bald, grouchy Sphynx cat who will bite you if you mistake him for a Chihuahua.
Just read the rules and isn't clear about a couple of things.
first. Can you buy more than one reinforcement in the same turn. In the rules it says "ask for reinforcments or build a settlement". reinforcments are in plural signaling that you get more than one.
My rules read, "Players cannot perform this action more than once during each development phase." I interpret this to meant that only one token (a set of reinforcements) may be built per turn.
second. Is it possible to recruit new explorers if there aren't any more of the neutral kind to remove when you get them?
Example. If there are 4 players, three of them natives and one of the settler tallying 4 neutral natives. Each time you recruit a native you have to remove a neutral native, but it's possible that they will deplete very soon for the three native players.
No, it is not possible. This is part of the game's faction balancing mechanism. This was answered (for me, in fact) by the designer in this thread:
Do available neutrals limit new explorers?