I was never able to have 6 salvagers online for the return to Kharak either. There is a way to grab all the assault frigates though
Send in a small fighter wing to distract the frigates while you salvage corvettes get into position. Grab the closest frigate with a pair, but for the others, just have one salvage corvette go after each one. One by itself can't lockdown a frigate and tow it home, instead, it will start spinning it. This will stop it from attacking crew pods, and if all of them are disrupted, they won't be able to try to snipe the salvage corvettes off their buddies (try to time them fairly close, as they will attack the salvage corvettes once they latch on).
You can be resourcing while this is going on to get enough for salvage corvette #6, and as soon as 1 and 2 are done hauling in the first frigate, they can go back out to help bring in another two--just make sure to repair them if they're damaged. Even with a corvette latched on, those assault frigates can do some damage if their main gun happens to be spun in line with any of your ships.
As for the other Homeworlds, I've played part of Cataclysm, but never finished. I haven't tried Homeworld 2.