[MilCom] Question on multi scanner hook ups

[Ron]

 Do directional couplers with -xx dB isolation taps help or hinder multi 
scanner hook ups to a single antenna, in regards to them introducing birdies 
or interference back up the antenna cable to the other scanners causing you 
to run the 'squelch' on the units at a higher setting than if connected 
individually to their own antennae? (Talk about run on sentences!)
     I was given several of these with F connectors (input, output and 4 
isolation taps on each one,) with the 4 taps on each one being the same 
'isolation' value of either -15, -20, -25 or -30dB, by a friend who does 
CATV installs. Units are marked 4 Way directional tap, 5-500 MHz.
    Price was right (free), but before I start  buying a bunch of F 
connectors and adapters to hook the scanners up to my present single ICOM 
R7000 Disconne and experiment, I figured I'd throw it out to this august 
group.
    And while you are all online here, would they be of any assistance 
isolating an RS-392 Short wave receiver from a Racal 6790 using a single 
shared HF feed line? Would they work on frequencies below 5 MHz? With 
degraded functionality?
    Should the unused taps/outputs be terminated with 75 ohm terminators?
    If you think I have any idea of what I am talking about, you are 
mistaken!
    TIA, Ron in Southern NH