Question about decimation/downsampling
The key point is that you remove any signal outside of the final bandwidth before downsampling to avoid aliasing. If you want to do this analog or digitally it doesn't really make a difference technically. From an engineering perspective, digital filtering is typically more economical. As for pitfalls/design tips - if you do use digital filtering keep in mind that you can save CPU cycles by not computing filtered output samples which will ultimately be thrown away by downsampling. 

So to conclude, design a 100Hz LPF FIR at the 25kHz sample rate, run the filter on every 50th sample and then throw all but those samples away.