In 2016-17 there there 887,566 boardings at Tarneit compared with 541,400 at Wyndham Vale, which is the latest data available.
Roughly speaking that would be about 5000 weekday boardings between them; even if all 5000 are in the peak 2 hours, if it's spread across 12 trains in that time we've still got oodles of room.
Any infill stations will probably draw most of their patronage away from the existing two when they get built.
There are still HUGE tracts of land in this area yet to be developed, hence the government's recent emphasis on their infrastructure spend out west. Moreover, the tiny blocks of land that are allocated for new houses these days the population is only going to BOOM.
Forget about Little River only having a miniscule population...that's going to change very soon.