This is a very good question and many hobbiests don't really knows what's truly going on with "benifical bacteria(s)".
This is how I am come to know it:
There are two main types of "BB" ( beneficial bacteria ).
One ingests ammonia and the other ingests nitrite, this is done like this:
"Nitrification in nature is a two-step oxidation process of ammonium (NH4+) or ammonia (NH3) to nitrate (NO3−) catalyzed by two ubiquitous bacterial groups. The first reaction is oxidation of ammonium to nitrite by ammonium oxidizing bacteria (AOB) represented by the "Nitrosomonas" species. The second reaction is oxidation of nitrite (NO2−) to nitrate by nitrite-oxidizing bacteria (NOB), represented by the Nitrobacter species." -Wikipedia explanation
Plain English:
Fish make ammonia (NH3), beneficial bacteria #1 eats ammonia and excretes nitrite (NO2-), beneficial bacteria #2 eat nitrite and excretes nitrate.
Question: is it possible to have less of one type of BB then another?
Yes it is very possible to have more of one then the other when a tank is being cycled or after medicating.
During the nitrogen cycle your ammonia will spike the BB#1 will grow and then your ammonia levels will drop to 0, then you will have a nitrite spike, your BB#2 will grow and your nitrite will drop to zero and you will then have a nitrate spike, thus a water change will be needed to bring your nitrate back to zero. Thus the nitrogen cycle is complete.