The stat blocks are similar, but the -2 Wisdom of the Monkey Goblin vs the -2 Strength of the standard goblin makes the former seem the better candidate for the class.
Nope. Both are goblinoids but goblinoid isn't a race. As such, they are different. It'd be nice they counted as goblins but it'd have to come out and say it for it to work that way. That said, ask your DM: they might let it slide.
How 'bout the Oversized Goblin from the Monster Codex?
They are an official sub-race of goblins: As such, you can take any and all goblin options with them [unless there are some that require small size].
The difference is that the Oversized Goblin just modifies the base race [altering size and stats] while the goblin monkey is presented as in independent race.