when you apply for your ATPL, you need 150hours PIC multi-crew
Nope.
When you apply for JAA ATPL, you have to be at leat 21 yrs old, you have to pass ATPL theory, your total flight time should be at leat 1500 hrs, 500 of which on MPA and 250 PiC, 150 of which
may be PiCUS. So it's may and not must. If you have 250 PiC hrs, be it in C-172, cub, seneca or pawnee, your PiCUS time might be zero and still you can get your ATPL if you meet other criteria. PiCUS was invented for the lucky ones who went straight from their integrated courses to RHS of some shiny 15t+ turbine equipment so they can get their ATPLs and need not put their co-piloting on hold for 2-3 months while they pound circuit in C-150 and build PiC time.
My CAA doesn't check logbooks for PiCUS entries. To have your PiCUS time recognized, you have to have letter of confirmation by your trainning dept. that you have indeed accrued necessary number of picus hours. As my company policy is to issue confirmations only for pilots who are to be upgraded shortly, F/O with 4000 hrs on MRJT and his ATPL still frozen is not an uncommon sight in my part of the universe.