I connected mine to my NMEA network about 6 years ago and have never had an issue. In fact I removed all my analog gauges and just use the Graphs and data from the NMEA network now. I had to add a electronic water pressure sensor to get the water pressure on the NMEA network and you have to choose if you want analog trim reading or if you want trim data on the NMEA network. I get RPM, Oil pressure, Water pressure, Engine Temperature, Manafold pressure, Fuel rate burn, Miles per gallon, engine hours, and output voltage.
I believe you are confused about the analog tach issue. The issue with gauge choice is just for the trim. There is a single sender for trim data and you have to choose to either send it to your analog gauge or send it to your ECU. The ECU certainly knows the RPM the engine is operating at. Your analog tach works off the ignition pulses and does not interfere with getting RPM data off the NMEA bus. In fact when I hooked my engine up to the NMEA bus I found out that my analog tach was not showing correct RPM. I replaced it and the new one is much closer to correct but is typically slightly off what the ECU shows.
I also added the lowrange fuel data storage device so that I could track fuel. When you fill up the boat you just go in and tell it you refilled and it will also show how much fuel is in the tank. I set an alarm at 15 gallons so that I know when to head for the gas station.
Once you get all the data available you can simply use the numerical overlay features on the HDS to overlay the info you want to see onto the map screen. You never have to look at guages.