There is no advantage or disadvantage to combo gauges, unless you have more than three areas you want to monitor and you are don't want to have additional mounting locations for the gauges (ie Dash, bullets, etc)
If you have a 24V auto and tow, right off the bat I suggest a minimum of 4 gauges- EGT, Boost, FP and transmission. If you only want a pillar mount, then that's the reason to go combo. I currently have 6 gauges, accross the a pillar and dash pod, and am going to add two more as I need them (for auxillary transmission). Deceide on what you like, and where you want to put them, and go for it.
Do a search for "FP gauge" or "Fuel Pressure gauge" and you should have more than enough reading. You'll get the "tastes great-less fillling" debate on whether electric or mechanical is better.....
There are also numerous threads with photos to be had for searching "gauge install" .
I would highly suggest calling Randy at
Diesel Injection Service , they have excellent service and prices on ISSPRO and Autometer gauges.
Disclaimer: No affiliation with DIS, Autometer, or ISSPRO. I just think they're great companies to deal with.
Kev