You could use a reducing busing like this
The 1.5in PVC would go in one one, and you would likely need a few inches of 1in PVC to go between the bussing and the bulkhead - that's assuming the bulkhead is Slip
Now in stock - 1-1/2" x 1" Sch 40 PVC Reducer Coupling Soc 429-211. Order today from PVC Fittings Online!
www.pvcfittingsonline.com
You could also us a reducing elbow as well like this - again, assuming the bulkhead is slip
1-1/2" x 1" Sch 40 PVC Reducing 90 Elbow Soc 406-211 in stock at PVC Fittings Online! Order today and we'll ship to you.
www.pvcfittingsonline.com
its actually a really good idea to run oversized pipe to your bulkhead - it also you to get more throughput to the outlets due te less friction and allows you utilize more of you pumps output capability