Yeah as a last resort I would think you could use them all as 'counts as'. Even the combi plasma-stub you could just count it as a separate plasma pistol / stub gun. Shock whip could be a shock maul, etc.
I think you could do some interesting things with them as new gear though. For example the chem-thrower could maybe automatically wound, or on a 6 if he has a respirator, and use the needle weapon injury rules. Stub cannon could be base Strength 4 with the option of dum-dums. Rivet cannon may work fine if you removed the move-or-fire restriction and increased the long range to 4-12. Simplest option for the combat shotgun could be to treat it as a regular shotgun but with a 12" short range. I haven't had too much chance to look over all their rules yet, but seems like it's a nice opportunity to add a bunch of interesting options with potentially many more down the road.