Yeah, but SHIELD doesn't know this immediately. The last time a threatening Asgardian came he killed hundreds of people, brought an army and threatened to take over the world. Nevermind that they are still suffering the effects of having to clean up the mess, etc. this is MCU, not comic book universe where things are treated differently and everyone is used to aliens coming and going. I understand that within the scope of the show she doesn't remotely seem like the big threat Loki was, but if I was running SHIELD you bet I'd take it as such.
While it's true they don't know what to expect, it could be either friend or foe. Which is why I believe Coulson's team was sent simply because they could be the first to respond. If the events of Winter Soldier are already happening at the same time, then it's quite possible that SHIELD's primary attention is there so they'd send a team to scout the possible danger before they'd mobilize and entire squad/batallion/whatever. I mean top tier or not a group of Humans would have no chance of actually stopping a real Asgardian threat on the level of Loki so to assume that Coulson's team was given that task to fight one off seems like you're underestimating the intelligence of SHIELD's higher command.
You have to prioritize threats and you can't take complete focus off a confirmed threat to check out a possibility of one. Also the film MCU has dealt with super powered threats many times by now. Hulk, Iron Man, Captain America,
And the last time only Thor came, again, against Loki and his army. Asgard obviously have an army too and should've felt at least partially responsible for Loki but they didn't send "a whole team" either, so I don't see how that point stands.
It's been awhile since I've seen it but I don't recall Loki having an Army in Avengers. At least not when he's first on Earth. But his arrival, even without a full army, prompted Fury to activate the Avengers initiative, using Banner, Iron Man, and Captain America to capture him. They only sent Thor because it was only Loki and Thor is capable of handling him. So even in that case, it's sending an appropriate response to the threat. No Chitauri Army was present on Earth when Thor was sent.
Edit: I kind of come across contradicting myself in that last paragraph. What I meant was, I don't recall Loki having an Army when he first arrives on Earth in Avengers.