A 125 is the recommended minimum size for most arrowanas. Liveaquaria.com recommends as much...
Silver:
http://www.liveaquaria.com/product/prod_Display.cfm?siteid=21&pCatId=858
Jardini:
http://www.liveaquaria.com/product/prod_Display.cfm?siteid=21&pCatId=859
They do get to 3 feet, so be aware that they will be VERY powerful fish when mature, and have several friends around to help you if you ever need to move them!!!!
125 should be fine for the arrowanas. They are flexible fish, and won't be stuck moving in place. Mind you, I think when they get that big, their motion in the tank will be quite limited. (Personally, I'd never get a fish that large without a 600 gallon aquarium--but my standards of humane-ness (humanity) are a little high!).
Reflect on this: arrowanas are surface swimmers, so the longer and wider the tank, the better. The height of the tank is of almost no consequence. You might be better with a longer, less capacious tank than a shorter, more capacious tank.
As to compatibility, I'm no expert.
Good luck. Hey, think about getting a Black Ghost Knife fish or an Ornate Bichir for a 125 gallon. They will be more comfortable in there. Here are a couple of links, but look them up on Fishbase.org for better pix:
Black Ghost Knife:
http://www.liveaquaria.com/product/prod_Display.cfm?siteid=21&pCatId=857
Ornate Bichir:
http://www.liveaquaria.com/product/prod_Display.cfm?siteid=21&pCatId=861
Let me know what you think about these fish (if you haven't already encountered them). I could also come up with some other suggestions for big fish if you'd like: i.e. gars, tropical eels.