Questions about whole house filters

When I bought my cottage, well water went through 2 filters, each 2.5"x10". One of the units cracked during the winter and I replaced it with a 4.5"x10" unit to cut down on the resistance. Last week I bought a 2.5"x20" unit at a garage sale for $1 and a sand separator at Lowes on closeout for $5. Questions: 1) I am thinking about pulling the 2.5"x10" and installing the the 2.5"x20" and the sand separator. I figure that the two new filters probably have no more resistance than the old one. Does that make sense, or is 3 filters just too many? (My filters last about 2 months and there is a lot of coarse sand in the water. I think a sand separator would dramatically improve the life of the other filters.) 2) The sand separator is made of acrylic and has a cap on the bottom to drain the sand. You can turn the water off and remove the cap, or replace the cap with a valve (1" IP thread) and drain with the water on; I expect a valve will be a lot easier and cleaner. But the manufacturer says it has to be a plastic valve, not metal. I happen to have an extra brass valve, but would have to buy a plastic one. (But couldn't even find a plastic one at HD today.) Why plastic? I can't ask the manufacturer cause they don't seem to exist anymore.
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