You can go slower on the pump, however there is a minimum recommended speed to operate at. Make sure what that is and the HP to turn that speed. Some of the manufactures put out a chart that will show pressure/speed/HP/cfm's. Make sure that you have the electric power to run it also. Your starting to talk about bit amps here.
on edit, here is an example from Kellogg for pressure/speed/HP/cfm's.
http://www.aircompeq.com/ka.htm
edit#2, here is from SB,
http://www.saylor-beall.com/pumps/ . If you have a 4500 pump then it looks to be spec'ed for 10-15 hp and may not do well with 5 -7 hp. Also on the SB site they have manuals for the different series pumps you may want to read once you figure out the pump your looking at.
lg
no neat sig line