Apply conditional SNATPOOL

I don't really think you need two separate iRules:

when CLIENT_ACCEPTED {
    if { [class match [IP::client_addr] equals DGROUP_THS] } {
        snatpool [class match -value [IP::client_addr] equals DGROUP_THS]
    } 
}

where your datagroup includes two different entries:

ltm data-group internal DGROUP-THS {
    records {
        10.100.6.64/27 { 
            data SNAT_THS
        }
        172.26.160.0.24 {
            data SNAT_DEFAULT
        }
    }
    type ip
}

Or if you just have a default SNAT for anything that's not in the 10.100.6.64/27 subnet:

when CLIENT_ACCEPTED {
    if { [IP::addr [IP:client_addr] equals 10.100.6.64/27] } {
        snatpool SNAT_THS
    } else {
        snatpool SNAT_DEFAULT
    }
}