Avanti found this link quite a while back. More then you ever needed to know about connecting different batteries in parallel:
SmartGauge Electronics - Interconnecting multiple batteries to form one larger bank
Bottom line: Internal resistance matters a great deal. When connected in parallel even relatively small differences will create significant differences in current loading between batteries. Your array will work, but some of the batteries will work harder than others and will wear out more quickly.
Possible solutions: As Rlum said, making sure you have a charging system that can periodically fully fill the cells and thus rebalance them is a good starting point. The next thing you could do would be to simply accept the differences in wear and recognize that the lifespan of your array will be shorter than the lifespan of your individual batteries. I would choose this option - LiFEPo4 prices have been dropping and this is the simplest and not particularly expensive option.
If you don't want to have reduced array lifespan then there is always the option of using a ammeter to measure the current flowing from each battery under load and manually balance them by adjusting the length of the wire used to connect them to the array so that the total resistance is more closely matched between batteries. This would not be perfect as I don't think internal resistance is constant, but you could balance it for the most typical withdrawl scenario and prolong the life of the otherwise most-stressed batteries. It wouldn't cost much to do this from a material perspective, but if you needed to hire someone skilled enough to do this well it would probably not be cost effective.