Bigger softboxes and bigger umbrellas, no problem. The umbrellas are standard fitting and the softbox fitting is 'Universal Speedring' which is basically a bolt-on, available from several manufacturers including Lencarta.
In terms of stability there's nothing between the spring stands and the air cushioned, but the air cushioned come into their own if you 'drop' the light by forgetting to support the weight when you adjust the height - instead of crashing down, the light comes to a gentle halt with a satisfying hiss of air
If you're going to buy a couple of stands and get a pole I think it would be cheaper to get the assembly.
As for accessories, no they don't take the same range as say Interfit, Bowens, Elinchrom and I see this as a weakness if you're planning to use them say for product photography. The reason I suggested the 3 head kit, apart from the better stands, is that a snoot, barn doors, gels, 2 honeycombs, an umbrella and various reflectors (I think) are all included and are all you need for portrait/glamour type use.
Hope this helps