yes, all processing is done in float values these days, except if the host called setProcessingPrecision (ProcessingPrecision precision), then it uses doubles i.e. 64 bit flaoting point, see: http://www.juce.com/doc/classAudioProcessor#ab9c8a901838ee7d35df06d849a1709a2
You can see in the example plugin a nice way, how you can template your processing for FloatType, so you don't have to code float and double variants separately: https://github.com/julianstorer/JUCE/blob/master/examples/audio%20plugin%20demo/Source/PluginProcessor.h
Yes, whenever you are in the context of DAWs, audio plug-ins, and JUCE apps, your audio will be represented as 32-bit floats with the valid values being in the range [-1.0, 1.0], regardless of what other formats are used elsewhere in your system.