Ideally, you would want to isolate each 4-20mA input. In practice, this can be quite expensive. It all depends on your intended application. For most applications, if your equipment is supplying the loop voltage it is not necessary to have each input isolated. When I need to do this I use modules from Pepperl & Fuchs which are expensive and for use in hazardous areas.

You could do some research on the web to see how PLC manufacturers recommend connecting to their equipment - ALLEN BRADLEY,SCHNEIDER,PEPPERL & FUCHS etc. Generally they will have a 250 ohm resistor, one end to 24vdc the other to the 4-20mA loop device and the differential inputs measure across the resistor. The other end of the 4-20mA loop device connects to 0V.

Normally the 4-20mA loop devices won't be totally floating - at some point the power supply will be connected to plant earth (except if it is a hazardous area device with a Exi galvanic isolator - in which case the safe area side will still be referenced to plant earth at some point).

I haven't read the specs for the 1 wire a/d and suspect it might be an idea to research other parts that may be more cost effective. May I suggest you peruse Analog Devices,Texas Instruments, Maxim ,Linear tech websites - there is a wealth of app notes on 4-20mA stuff. As for isolating 1wire - it should be pretty simple, if you're asking me how to do it, maybe you shouldn't be attempting this project! Adapting the RS232 one should be simple - I expect you'll need two port pins on your micro though - one for send, the other for receive. In this application, the PC900v may not be the best choice.

Do some research, do some experimentation and your questions will be answered.