Loco allotment is usually based on loco availability in the trip shed, and in some cases like trains with extremely aggressive schedules, allotment may be based on schedule requirements. But more often than not, it's based on loco availability.
Also, even if a train is a 'Super slow crawler', it's because the train has too many stops. Even crawlers have regular runs at MPS in between stations. It's not like they are made to run at lesser speed. they just have most stops/less priority which makes their run time longer. Both the 'fast' trains and the 'slow' trains run at 110kmph in some sections during their run.That way it's justified that they get normal passenger locos like...
more... WAP4.
Most important thing to note is that in IR, locomotives never determine the run time of a train. Other than the Bhopal Shatabdhi which absolutely needs 150Kmph runs to maintain it's schedule and probably some Raj/Duronto trains which needs higher MPS to maintain schedules, All normal SF/Express can comfortably maintain their schedules with P4/WAM4 and if the train does not have too aggressive a schedule, even with WAG5/7/9 locos.
For example, WAP4 can comfortably maintain 75+ kmph average speed trains. With moderately aggressive schedules, WAM4 can also maintain 70kmph Average speeds. And with normal schedules, WAG5/7 can maintain 60+kmph average speed trains. Even if you consider the MPS, WAP4/P7 can reach 140kmph, WAM4 ~100kmph and WAGs can run at 90kmph comfortably and ICF coaches have MPS of only 110kmph. Even if you have a loco that can go more than 110, it's of no use.
Most of the trains in IR have < 60kmph average speed. But all of IR locos are capable of much better runs.The slow schedules are because of congestion, track over-utilization, poor controlling etc and NOT because the locos are capable. In the current situation, other than a few trains, for most of the trains in IR, it doesn't make a difference as to which loco is used to haul the train.
Trains with more halts will need locos with better acceleration , that's why P4/P7 are most important for crawlers. For trains with less stops, WAP4/WAM4 can haul them comfortably without any schedule impact. It'll be a waste if a P7 is used for a Duronto and a WAM4 is used for a crawler, not the other way round.