A total of 400 XP is a CR 1 encounter, so yes, creatures adding up to 400 XP is right for a normal encounter for that party.
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APL of your players should equal the CR of an encounter.
In your example, APL is 1 and CR 1 encounter would be an average encounter per guidelines. In order to create a CR 1 encounter, you take the 400 XP and manipulate it. For example a duo of badgers would be CR 1 encounter as they both grant 200 XP each.
An APL +2 would be CR 3 encounter in your case, and much harder to go against.
Hope this helps,
Adam
First of all there's no hard limit on the amount of monsters you can have. It depends on how difficult you want the encounter to be. For a party of 4 level 1 characters, a CR 1 encounter will be a mild challenge, a threat without being particularly difficult. Often times encounters are one two or even 3/4 higher then the average party level if they are meant to be difficult. Keep in mind also, though the game guidelines treat 4 and 5 players the same, I personally dont agree with that, if there are 5 members in my party I treat the avergate party level as 1 higher then normal (treating them as a level 2 party).
Also as a side bit of advice, it is usually better to increase the difficulty of encounters by adding more monsters, not by taking one more powerful monster, due to the action economy, and the amount of threat the monster would be to any one party member.
Also, at low levels, CR can be misleading. Not all CR 1 encounters are created equal. Some are very dangerous, while others are nearly mpossible to fail. As the GM, choose accordingly, and feel free to adjust the CR if you find something particularly broken with the encounter. (Or just fix the encounter to be less broken.)