Don't take these alignments too seriously! They are just a help for the player to decide wich way his charakter would react in a certain situation... no more. For sure a chaotic charakter can be a loyal and helpful member of a party. He just decides it is the right thing to support the others, or he just likes them bunch, or it helps him to follow his private aims... An evil charakter also can be a valuable member of a party... as long his own private interests have the same goal as the rest of the group... no problem in this. Being evil doesn`t automaticly mean this charakter will kill everything and everyone in sight... but that he will normally act greedy, selfish and without scruples.

And having a party with very differing alignments adds spice to the roleplay, for the players have to communicate a lot between the charakters in order to check positions over and over.