But to answer your question, if you feel that your minion (not your familiar) has a significant effect on your killing power, I wouldn't recommend hiring a second minion. If it is just a meatbag or fodder for your opponents, then I would recommend hiring a second minion.
Another thing: If your first minion doesn't really last until melee, then I would recommend untraining DM on him, and dumping it on the second minion if you intend to keep DM at all. That way, you have a nice suicide bomber.
BUT, there is a catch... If the HP added by the AS you train on your enchanter makes it so your fireball minion does last until melee, and beyond, it might do more harm than good.
Now for the minion order. Well, You could have your enchanter up front to suck up some damage, fireball in the middle and familiar in the back, although it will be susceptible to magic missile. If you want to avoid that, You might want to put your fireball in the back and equip a small weapon on him so the familiar spawns in the middle. However, this setup makes it so your fireball minion is likely to last the longest (except against teams that use magic missile), which might not be a great thing because fireball has friendly fire turned ON. (=P)
The best order I can see is fireball up front, followed by the familiar, and the enchanter in the back to absorb the magic missile damage. This ensures that the fireball minion continues to be a suicide bomber, and your familiar won't be prone to magic missile for quite some time, because of your meatbag enchanter is in the back taking the beating for your familiar. Your familiar will most likely survive the longest, which is a good thing.