RFID Logger & Battery
The specs state 'maximum rate' for the set-up I am using is 88mA. I
  plan on using a 12V battery. It is really as simple as 0.088 x 24 = I
  need at least 2.1 AH in a battery? So with a 6AH battery, I should get
  ~68 hours of use?


Theoretically that's correct, but in practice the battery cannot be expected to deliver its full rated capacity. Firstly the capacity declines as the battery ages, and secondly over-discharging will damage it. You must ensure that the battery is recharged before it becomes completely discharged. Depending on the installation and access, the realizable capacity could be significantly less than 6 Ah. 

Cycle life also reduces with greater depth of discharge (DOD). A battery with double the capacity could provide more than twice as many cycles, as well as being less vulnerable to over-discharge. Finally, shelf life reduces at lower state of charge. Shallower discharges keep the battery at a higher average charge level.

The usual recommendation is to never go below 50% DOD, so you should specify double the required capacity. However this has to be balanced against cost and required lifespan. At 100% DOD the battery might do 150-200 cycles, but a few slight over-discharges might reduce that to single digits. 


  Also, the unit specs say it can run off 6V-13.2V input. Would be any
  benefit to using a step-down buck convertor to reduce voltage


The unit appears to have linear on-board regulators - so yes, using a step-down converter would be of benefit since it would waste less power and provide longer run time. Assuming 12 V in and 6 V out, the battery could last nearly twice as long. A 6 Ah battery should be able to do ~60 hours with 50% reserve capacity.

The buck converter will have a quiescent current draw that reduces efficiency at low current. You should chose a converter that has high efficiency at around 88 mA.


  I've had problems in the past with regulators interfering with radio
  signals, and the goal is to maximum the read-range of the antennas for
  134khZ RFID tags.


It might be an issue, but probably not if the regulator is positioned away from the rfid coils. Preferably use a regulator which switches at above 134 kHz.


  Finally, is the best way to connect it just to solder wires to the V+
  and V- inputs? Or is it better to use some kind of socket-like
  adaptor?


Soldered wires are more reliable (if properly soldered), but a connector is more convenient if you ever need to disconnect the cable for some reason. If the environment introduces significant vibration or moisture then soldering is recommended, though it would be better to eliminate those effects.